Lithium-ion secondary battery

ABSTRACT

A lithium-ion secondary battery ( 100 A) includes a positive electrode current collector ( 221 A) and a positive electrode active material layer ( 223 A) retained on the positive electrode current collector ( 221 A). The positive electrode active material layer ( 223 A) contains positive electrode active material particles, a conductive agent, and a binder. The positive electrode active material particles ( 610 A) each include a shell portion ( 612 ) made of primary particles ( 800 ) of a layered lithium-transition metal oxide, a hollow portion ( 614 ) formed inside the shell portion ( 612 ), and a through-hole ( 616 ) penetrating through the shell portion ( 612 ). The primary particles ( 800 ) of the lithium-transition metal oxide have a major axis length of less than or equal to 0.8 μm in average of the positive electrode active material layer ( 223 A).

TECHNICAL FIELD

The present invention relates to lithium-ion secondary batteries.

In the present description herein, the term “secondary battery” refersto a repeatedly chargeable storage device in general, and it is a termthat encompasses what is called storage batteries, such as lithium-ionsecondary batteries, nickel-metal hydride batteries, and nickel-cadmiumrechargeable batteries, as well as electrical storage elements such aselectric double-layer capacitors.

In the present description, the term “lithium ion secondary battery”refers to a secondary battery in which lithium ions are used aselectrolyte ions and charging and discharging are implemented by thetransfer of electrons accompanying lithium ions between positive andnegative electrodes. The battery commonly referred to as “lithiumsecondary battery” is a typical example encompassed by the lithium-ionsecondary battery of the present description. The present application isa divisional of U.S. application Ser. No. 14/362,660, which is anational phase application of International Application No.PCT/JP2012/081310, filed Dec. 4, 2012 and claims priority under theParis Convention, or under laws and regulations in the designatedcountries, from Japanese Patent Application No. 2011-266319, filed onDec. 5, 2011, the content of all of which is incorporated herein byreference.

BACKGROUND ART

Regarding a lithium-ion secondary battery, JP 2004-253174 A, forexample, discloses the use of, as a positive electrode active material,a layered lithium-transition metal oxide comprising a hollow particlehaving an outer shell portion and a space portion inside the outer shellportion. The lithium-transition metal compound oxide proposed therein issuch that, when it is cross-sectioned, the proportion of the area of thespace portion with respect to the total of the areas of the outer shellportion and the space portion is greater than 0% but less than 20%. Thepublication states that the use of such a lithium-transition metal oxidefor the positive electrode active material makes it possible to providea non-aqueous electrolyte secondary battery that shows excellent batteryperformance even under more severe use environments.

The manufacturing method of the layered lithium-transition metal oxideis disclosed in paragraphs 0026 to 0042 of the publication. Themanufacturing method disclosed therein roughly includes the followingprocedures. First, an aqueous solution containing cobalt ions and nickelions in a predetermined composition ratio is dropped in pure water thatis being agitated. Next, sodium hydroxide is dropped therein so that thepH will be 8 to 11, and cobalt and nickel are coprecipitated at atemperature of 40° C. to 80° C. at a number of revolution of 500 rpm to1500 rpm, to obtain a coprecipitated substance. Next, the obtainedcoprecipitated substance is filtered, washed with water, and thereafterdried, and then mixed with lithium hydroxide. The mixture is then bakedat a temperature of 650° C. to 1100° C. for 1 hour to 24 hours in anatmosphere in which the oxygen partial pressure is controlled, tosynthesize a lithium-transition metal oxide.

JP 2011-119092 A discloses active material particles for a lithiumsecondary battery. The active material particles disclosed thereinconstitute a hollow structure having a secondary particle, in which aplurality of primary particles of a lithium-transition metal oxide areaggregated, and a hollow portion formed therein. It is also proposedthat the secondary particle has a through-hole penetrating from theoutside into the hollow portion, and that the BET specific surface areathereof is set to from 0.5 m²/g to 1.9 m²/g. The publication states thatsuch a hollow active material particle can achieve an improvement inhigh-rate performance, an improvement in durability, prevention ofresistance increase, and an improvement in capacity retention ratio atthe same time.

JP 3032757 B discloses the use of, as a positive electrode activematerial, a composite oxide represented by the general formulaLi_(x)M_(1-y)A_(y)F_(z)O_(2n-z), (where 0.9≦x≦1.1, 0≦y≦0.5, 0≦z≦0.25,1≦n≦2, M is at least one transition element selected from the groupconsisting of Co, Ni, and Mn, and A is at least one element selectedfrom the group consisting of Co, Ni, Mn, B, and Al), although it isunclear whether the positive electrode active material contains such ahollow particle as described above. The just-mentioned patent statesthat the positive electrode active material forms secondary particleseach made of primary particles having a crystal structure with C-axisorientation tendency, and that the ratio (D₅₀/r) of the particle sizeD₅₀, at which the cumulative volume of the secondary particles reaches50% in particle size distribution, to the average shorter axis length rof the primary particles is 10≦(D₅₀/r)≦50. According to the patent, itprovides a non-aqueous electrolyte secondary battery that shows highdischarge voltage, excellent charge-discharge characteristics at highcurrent, and excellent cycle performance.

CITATION LIST Patent Literature

[Patent Literature 1] JP 2004-253174 A

[Patent Literature 2] JP 2011-119092 A

[Patent Literature 3] JP 3032757 B

SUMMARY OF INVENTION Technical Problem

Lithium-ion secondary batteries have been increasingly used as thebatteries for driving hybrid vehicles, which require high levels ofoutput power characteristics at high rate and cycle performance, andmoreover as the batteries for driving plug-in hybrid vehicles andelectric vehicles, which require a particularly high level of capacity.The present inventor believes desirable that the lithium-ion secondarybatteries used as such batteries can exhibit high power stably even atlow charge levels. Herein, it is proposed to provide a lithium-ionsecondary battery that makes use of the hollow active material particleas disclosed in JP 2011-119092 A as the positive electrode activematerial particle, and that can improve output power particularly at lowcharge levels (i.e., in a low SOC region).

Solution to Problem

A lithium-ion secondary battery according to one embodiment of thepresent invention comprises a current collector and a positive electrodeactive material layer retained on the current collector. The positiveelectrode active material layer includes positive electrode activematerial particles, a conductive agent, and a binder. The positiveelectrode active material particles each comprise a shell portioncomprising primary particles of a layered lithium-transition metaloxide, a hollow portion formed inside the shell portion, and athrough-hole penetrating through the shell portion. In addition, theprimary particles of the lithium-transition metal oxide have a majoraxis length of less than or equal to 0.8 μm in average of the positiveelectrode active material layer. This lithium-ion secondary battery canimprove the output power particularly at low charge levels.

The major axis length of the primary particles of the lithium-transitionmetal oxide may be equal to or greater than 0.2 μm. This ensures thatthe positive electrode active material particles have required strength.The through-hole may have an aperture width of from 0.01 μm to 2.0 μm inaverage of the positive electrode active material layer.

It is also possible that the proportion of the hollow portion may beequal to or greater than 23% of the apparent cross-sectional area of thepositive electrode active material particle, in average of the positiveelectrode active material layer. It is also possible that, when thethickness of the shell portion at an arbitrary position within an innersurface of the shell portion is defined by the minimum distance from thearbitrary position within the inner surface of the shell portion to anouter surface of the shell portion in an arbitrary cross section of thepositive electrode active material layer, the thickness of the shellportion may be less than or equal to 2.2 μm in average of the positiveelectrode active material layer. This makes it possible to suppress thediffusion resistance of lithium ions in the positive electrode morereliably, and to maintain high output power of the lithium-ion secondarybattery at a low charge level and in a low temperature environment, forexample. It is also possible that the thickness of the shell portion maybe equal to or greater than 0.1 μm. This ensures that the lithium-ionsecondary battery has required strength.

It is also possible that the lithium-transition metal oxide may containat least nickel, cobalt, and manganese as its constituent elements. Itis also possible that the lithium-transition metal oxide mayadditionally contain tungsten. It is also possible that the tungsten maybe contained in the lithium-transition metal oxide in an amount of from0.05 mol % to 2.0 mol % relative to the amount of the transition metals.This makes it possible to obtain the positive electrode active materialparticles in which the primary particles of the lithium-transition metaloxide have a major axis length of less than or equal to 0.8 μm morereliably.

It is preferable that the positive electrode active material particlesbe manufactured by, for example, a method comprising the steps of:mixing a lithium compound and a transition metal hydroxide containing atleast one transition metal element constituting the lithium-transitionmetal oxide, to prepare an unsintered mixture; and sintering the mixtureto obtain active material particles.

In this case, it is preferable that, for example, in the step of mixing,the unsintered mixture contain tungsten in an amount of from 0.05 mol %to 2.0 mol % relative to the amount of other transition metal(s). It isalso preferable that, for example, in the step of producing a sourcehydroxide, an aqueous solution of a transition metal compound containingtungsten be produced, and a transition metal hydroxide containingtungsten be obtained in the form of particles of the transition metalhydroxide. In this case, it is also preferable that the transition metalhydroxide be allowed to contain tungsten in an amount of from 0.05 mol %to 2.0 mol % relative to the amount of other transition metal(s).Furthermore, it is also preferable that in the step of producing asource hydroxide, an aqueous solution Aq_(A) containing at least oneelement of Ni, Co, and Mn be prepared, an aqueous solution Aq_(C)containing tungsten be prepared separately, and the aqueous solutionAq_(A) and the aqueous solution Aq_(C) be mixed under an alkalinecondition to produce an aqueous solution of a transition metal compoundcontaining the tungsten. It is also preferable that the lithium compoundbe lithium carbonate. In addition, the step of sintering may preferablybe performed, for example, at a sintering temperature of from 750° C. to950° C. in an air atmosphere.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating one example of the structure of alithium-ion secondary battery.

FIG. 2 is a view illustrating a wound electrode assembly of thelithium-ion secondary battery.

FIG. 3 shows a cross section taken along line in FIG. 2.

FIG. 4 is a cross-sectional view illustrating the structure of apositive electrode mixture layer.

FIG. 5 is a cross-sectional view illustrating the structure of anegative electrode mixture layer.

FIG. 6 is a side view illustrating a portion where an uncoated portionof the wound electrode assembly is welded to an electrode terminal.

FIG. 7 is a view schematically illustrating a state during charge of thelithium-ion secondary battery.

FIG. 8 is a view schematically illustrating a state during discharge ofthe lithium-ion secondary battery.

FIG. 9 is a view illustrating a lithium-ion secondary battery accordingto one embodiment of the present invention.

FIG. 10 is a view illustrating a wound electrode assembly of alithium-ion secondary battery according to one embodiment of the presentinvention.

FIG. 11 is a view illustrating the structure of a positive electrodeactive material layer of the lithium-ion secondary battery.

FIG. 12A is an electron micrograph of positive electrode active materialparticles in the positive electrode active material layer.

FIG. 12B is an electron micrograph of positive electrode active materialparticles in the positive electrode active material layer.

FIG. 13 is a cross-sectional SEM image of a positive electrode activematerial particle (secondary particle) in the positive electrode activematerial layer.

FIG. 14 is a schematic view illustrating a primary particle of thepositive electrode active material particle.

FIG. 15 is a schematic view illustrating a crystallite of the positiveelectrode active material particle.

FIG. 16 illustrates the relationship between the minor axis length (μm)of primary particles and the 003 plane crystallite size (Å) of thelithium-transition metal oxide in the positive electrode active materialparticles.

FIG. 17 illustrates the relationship between the major axis length ofprimary particles of the lithium-transition metal oxide in the positiveelectrode active material particles and the output power characteristicsat a low charge level.

FIG. 18 is a view illustrating a vehicle incorporating a secondarybattery.

FIG. 19 is a graph illustrating a fitted curve for calculating outputpower characteristics 1.

DESCRIPTION OF EMBODIMENTS

Here, an example of the structure of a lithium-ion secondary battery asa non-aqueous electrolyte secondary battery will be described first.Then, referring to the example of the structure as appropriate, alithium-ion secondary battery according to one embodiment of the presentinvention will be described. The parts and components that exhibit thesame workings are denoted by the same reference symbols as appropriate.The drawings are depicted schematically and do not necessarily reflectactual objects. The drawings merely show examples, and they do not limitthe invention unless otherwise stated.

FIG. 1 illustrates a lithium-ion secondary battery 100. As illustratedin FIG. 1, the lithium-ion secondary battery 100 has a wound electrodeassembly 200 and a battery case 300. FIG. 2 is a view illustrating thewound electrode assembly 200. FIG. 3 shows a cross section taken alongline in FIG. 2.

As illustrated in FIG. 2, the wound electrode assembly 200 has apositive electrode sheet 220, a negative electrode sheet 240, andseparators 262 and 264. The positive electrode sheet 220, the negativeelectrode sheet 240, and the separators 262 and 264 are strip-shapedsheets.

<<Positive Electrode Sheet 220>>

The positive electrode sheet 220 has a strip-shaped positive electrodecurrent collector 221 and a positive electrode active material layer223. A metal foil suitable for the positive electrode may be usedpreferably for the positive electrode current collector 221. For thepositive electrode current collector 221, it is possible to use, forexample, a strip-shaped aluminum foil having a predetermined width and athickness of about 15 μm. In the example shown in the drawings, anuncoated portion 222 is provided along one lateral-side edge of thepositive electrode current collector 221. As illustrated in FIG. 3, thepositive electrode active material layer 223 is retained on both facesof the positive electrode current collector 221 except for the uncoatedportion 222, which is provided in the positive electrode currentcollector 221. The positive electrode active material layer 223 containsa positive electrode active material. In this embodiment, the positiveelectrode mixture layer 223 is formed by coating a positive electrodemixture containing the positive electrode active material onto thepositive electrode current collector 221.

<<Positive Electrode Active Material Layer 223>>

Here, FIG. 4 is a cross-sectional view of the positive electrode sheet220. In FIG. 4, positive electrode active material particles 610,conductive agent 620, and binder 630 in the positive electrode activematerial layer 223 are enlarged schematically so that the structure ofthe positive electrode active material layer 223 can be shown clearly.As illustrated in FIG. 4, the positive electrode active material layer223 contains the positive electrode active material particles 610, theconductive agent 620, and the binder 630.

<<Positive Electrode Active Material Particles 610>>

Various types of substances that can be used as the positive electrodeactive material of lithium-ion secondary batteries may be used for thepositive electrode active material particles 610. Examples of thesubstances for the positive electrode active material particles 610include lithium-transition metal oxides, such as LiNiCoMnO₂(lithium-nickel-cobalt-manganese composite oxide), LiNiO₂ (lithiumnickel oxide), LiCoO₂ (lithium cobalt oxide), LiMn₂O₄ (lithium manganeseoxide), and LiFePO₄ (lithium iron phosphate). Here, LiMn₂O₄ may have,for example, a spinel structure. LiNiO₂ and LiCoO₂ may have a layeredrock-salt structure. LiFePO₄ may have, for example, an olivinestructure. The LiFePO₄ with an olivine structure may include, forexample, particles in the range of nanometers. The LiFePO₄ with anolivine structure may further be coated with a carbon film.

<<Conductive Agent 620>>

Examples of the conductive agent 620 include carbon materials, such ascarbon powder and carbon fiber. As the conductive agent 620, it ispossible to use one of the just-mentioned examples of the conductiveagents either alone or in combination with another one or more of theexamples. Examples of the carbon powder include various types of carbonblacks (such as acetylene black, oil-furnace black, graphitized carbonblack, carbon black, graphite, and Ketjen Black) and graphite powder.

<<Binder 630>>

The binder 630 serves to bond the particles of the positive electrodeactive material particles 610 and the conductive agent 620 contained inthe positive electrode active material layer 223 with each other, and tobond these particles with the positive electrode current collector 221.As the binder 630, it is possible to use polymers that can be dissolvedor dispersed in the solvent used. For example, for the positiveelectrode mixture composition using an aqueous solvent, it is preferableto use water-soluble or water-dispersible polymers, including:cellulose-based polymers (such as carboxymethylcellulose (CMC) andhydroxypropyl methyl cellulose (HPMC)); fluoropolymers (such aspolyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), andtetrafluoroethylene-hexafluoropropylene copolymer (FEP)); and rubbermaterials (such as vinyl acetate copolymer, styrene-butadiene copolymer(SBR), acrylic acid-modified SBR resin (SBR latex)). For the positiveelectrode mixture composition using a non-aqueous solvent, it ispreferable to use polymers (such as polyvinylidene fluoride (PVDF),polyvinylidene chloride (PVDC), and polyacrylonitrile (PAN)).

<<Thickening Agent and Solvent>>

The positive electrode active material layer 223 is formed, for example,in the following manner a positive electrode mixture is prepared bymixing the positive electrode active material particles 610 and theconductive agent 620 mentioned above into a paste form (slurry form) ina solvent, and the positive electrode mixture is then coated onto thepositive electrode current collector 221, dried, and pressure-rolled. Inthis case, either an aqueous solvent or a non-aqueous solvent can beused as the solvent for the positive electrode mixture. A preferableexample of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). Theabove-mentioned examples of the polymer materials used as the binder 630can also be used for the purpose of obtaining the function as anaddition agent, such as a thickening agent for the positive electrodemixture, in addition to the function as the binder.

It is preferable that the mass ratio of the positive electrode activematerial in the entire positive electrode mixture be about 50 wt. % ormore (typically from 50 wt. % to 95 wt. %), and generally morepreferably from about 70 wt. % to about 95 wt. % (e.g., from 75 wt. % to90 wt. %). The proportion of the conductive agent in the entire positiveelectrode mixture may be from about 2 wt. % to about 20 wt. %, andgenerally preferably from about 2 wt. % to about 15 wt. %. In acomposition that uses a binder, the proportion of the binder in theentire positive electrode mixture may be from about 1 wt. % to about 10wt. %, and generally preferably from about 2 wt. % to about 5 wt. %.

<<Negative Electrode Sheet 240>>

As illustrated in FIG. 2, the negative electrode sheet 240 has astrip-shaped negative electrode current collector 241 and a negativeelectrode active material layer 243. A metal foil suitable for thenegative electrode may be used preferably for the negative electrodecurrent collector 241. For the negative electrode current collector 241,it is possible to use, for example, a strip-shaped copper foil having apredetermined width and a thickness of about 10 μm. In the example shownin the drawings, an uncoated portion 242 is provided along onelateral-side edge of the negative electrode current collector 241. Asillustrated in FIG. 3, the negative electrode active material layer 243is retained on both faces of the negative electrode current collector241 except for the uncoated portion 242, which is provided in thenegative electrode current collector 241. The negative electrode activematerial layer 243 contains at least a negative electrode activematerial. In this embodiment, the negative electrode active materiallayer 243 is formed by coating a negative electrode mixture containingthe negative electrode active material onto the negative electrodecurrent collector 241.

<<Negative Electrode Active Material Layer 243>>

FIG. 5 is a cross-sectional view of the negative electrode sheet 240. Asillustrated in FIG. 5, the negative electrode active material layer 243contains negative electrode active material particles 710, a thickeningagent (not shown), a binder 730, and so forth. In FIG. 5, the negativeactive material particles 710 and the binder 730 in the negativeelectrode active material layer 243 are enlarged schematically so thatthe structure of the negative electrode active material layer 243 can beshown clearly.

<<Negative Electrode Active Material Particles 710>>

As the negative electrode active material particles 710, it is possibleto use any conventional material used for lithium-ion secondarybatteries, either alone or in combination, without any particularlimitation. Examples of the negative electrode active material particles710 include particulate carbon materials (carbon particles) at leastpartially containing a graphite structure (a layered structure). Thenegative electrode active material particles 710 may be, for example,natural graphite, natural graphite coated with amorphous carbonmaterial, graphitic materials (graphites), non-graphitizable carbons(hard carbons), graphitizable carbons (soft carbons), and combinationsthereof. Here, the figure depicts a case in which what is called flakegraphite is used as the negative electrode active material particle 710,but the negative electrode active material particle 710 is not limitedto the example shown in the figure.

Other possible examples of the negative electrode active materialinclude metallic compounds (preferably metal oxides) containing metalelements such as Si, Ge, Sn, Pb, Al, Ga, In, As, Sb, and Bi. It is alsopossible to use LTO (lithium titanium oxide) as the negative electrodeactive material particles. For the negative electrode active materialcomprising a metallic compound, it is also possible that the surface ofthe metallic compound may be coated sufficiently with, for example, acarbon film to form a particulate material with good electricalconductivity. In this case, it is possible that the negative electrodeactive material layer contains no conductive agent, or that the contentof the conductive agent may be lowered relative to the conventionalnegative electrode active material layer. Such additional features ofthe negative electrode active material and forms thereof, such asparticle size, may be selected as appropriate according to the requiredcharacteristics.

<<Thickening Agent and Solvent>>

The negative electrode active material layer 243 is formed, for example,in the following manner a negative electrode mixture is prepared bymixing the negative electrode active material particles 710 and thebinder 730 mentioned above into a paste form (slurry form) in a solvent,and the negative electrode mixture is then coated onto the negativeelectrode current collector 241, dried, and pressure-rolled. In thiscase, either an aqueous solvent or a non-aqueous solvent can be used asthe solvent for the negative electrode mixture. A preferable example ofthe non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). For the binder730, any of the polymer materials shown as the examples of the binder630 of the above-described positive electrode active material layer 223(see FIG. 4) may be used. The previously-mentioned examples of thepolymer materials used as the binder 630 of the positive electrodeactive material layer 223 can also be used as an addition agent for thepurpose of obtaining necessary functions other than the function as thebinder, for example, as a thickening agent for the positive electrodemixture or the negative electrode mixture.

<<Separators 262 and 264>>

Each of the separators 262 and 264 is a member for separating thepositive electrode sheet 220 and the negative electrode sheet 240 fromeach other, as illustrated in FIGS. 1 and 2. In this example, each ofthe separators 262 and 264 is made of a strip-shaped sheet having aplurality of micropores and having a predetermined width. For theseparators 262 and 264, it is possible to use, for example, a singlelayer separator or a multi-layered separator, which is made of porouspolyolefin-based resin. In this example, as illustrated in FIGS. 2 and3, the width b1 of the negative electrode active material layer 243 isslightly wider than the width a1 of the positive electrode activematerial layer 223. In addition, the width c1, c2 of the separators 262and 264 is slightly wider than the width b1 of the negative electrodeactive material layer 243 (c1, c2>b1>a1).

In the example shown in FIGS. 1 and 2, each of the separators 262 and264 is made of a sheet-shaped member. Each of the separators 262 and 264should be a member that insulates the positive electrode sheet 223 andthe negative electrode sheet 243 from each other and at the same timepermits transfer of electrolyte. Therefore, the separators are notlimited to sheet-shaped members. In place of the sheet-shaped member,each of the separators 262 and 264 may be made of, for example, a layerof insulative particles that are formed on a surface of the positiveelectrode active material layer 223 or the negative electrode activematerial layer 243. The insulative particles may be made of aninsulative inorganic filler (for example, a filler of metal oxide ormetal hydroxide) or insulative resin particles (for example, particlesof polyethylene or polypropylene).

In this wound electrode assembly 200, as illustrated in FIGS. 2 and 3,the positive electrode sheet 220 and the negative electrode sheet 240are stacked with the separators 262 and 264 interposed therebetween, sothat the positive electrode sheet 220 and the negative electrode sheet240 face each other. More specifically, the positive electrode sheet220, the negative electrode sheet 240, and the separators 262 and 264are stacked in the wound electrode assembly 200 in the following order:the positive electrode sheet 220, the separator 262, the negativeelectrode sheet 240, and the separator 264.

In this case, the positive electrode active material layer 223 and thenegative electrode active material layer 243 face each other while theseparators 262 and 264 are interposed therebetween. The portion of thepositive electrode current collector 221 on which the positive electrodeactive material layer 223 is not formed (i.e., the uncoated portion 222)protrudes from one side end of the region where the positive electrodeactive material layer 223 and the negative electrode active materiallayer 243 face each other. The portion of the negative electrode currentcollector 241 on which the negative electrode active material layer 243is not formed (i.e., the uncoated portion 242) protrudes from theopposite end to the side end from which the uncoated portion 222protrudes.

<<Battery Case 300>>

In this example, as illustrated in FIG. 1, the battery case 300 is whatis called a prismatic battery case, and it includes a case main body 320and a lid 340. The case main body 320 has a closed-bottom quadrangularprismatic tubular shape, and is a flat-box-shaped case and whose oneside face (upper face) is open. The lid 340 is a member that is attachedto the opening of the case main body 320 (the opening in the upper facethereof) to close the opening.

For a secondary battery used for a vehicle, it is desired to improve theweight energy efficiency (the capacity of the battery per unit weight)in order to improve the fuel consumption of the vehicle. In thisembodiment, a lightweight metal such as aluminum or an aluminum alloy(aluminum in this example) is employed for the case main body 320 andthe lid 340, which constitute the battery case 300. This enables toimprove the weight energy efficiency.

The battery case 300 has a flat rectangular internal space as the spacefor accommodating the wound electrode assembly 200. As illustrated inFIG. 1, the width of the flat internal space of the battery case 300 isslightly wider than the width of the wound electrode assembly 200. Inthis embodiment, the battery case 300 has the case main body in aclosed-bottom quadrangular prismatic tubular shape and the lid 340closing the opening of the case main body 340. To the lid 340 of thebattery case 300, electrode terminals 420 and 440 are attached. Theelectrode terminals 420 and 440 penetrate through the battery case 300(the lid 340) and stick out outside the battery case 300. The lid 340 isprovided with a filling port 350 and a safety vent 360.

As illustrated in FIG. 2, the wound electrode assembly 200 is pressedand deformed into a flat shape in one direction orthogonal to thewinding axis WL. In the example shown in FIG. 2, the uncoated portion222 of the positive electrode current collector 221 and the uncoatedportion 242 of the negative electrode current collector 241 are exposedfrom the respective sides of the separators 262 and 264 in a spiralshape. As illustrated in FIG. 6, in this embodiment, the intermediateportions 224 and 244 of the uncoated portions 222 and 242 are gatheredand welded to the foremost end portions 420 a and 440 a of therespective electrode terminals 420 and 440. In this case, from theviewpoint of the difference in the materials of the positive electrodecurrent collector 221 and the negative electrode current collector 241,ultrasonic welding, for example, is used for welding the electrodeterminal 420 to the positive electrode current collector 221. On theother hand, resistance welding, for example, is used for welding theelectrode terminal 440 to the negative electrode current collector 241.Here, FIG. 6 is a side view illustrating the portion where theintermediate portion 224 (244) of the uncoated portion 222 (242) of thewound electrode assembly 200 is welded to the electrode terminal 420(440), and it is also a cross-sectional view taken along line VI-VI inFIG. 1.

The wound electrode assembly 200 is attached to the electrode terminals420 and 440 fixed to the lid 340 while it is pressed into a flat shape.As illustrated in FIG. 1, the wound electrode assembly 200 isaccommodated in the flat internal space of the case main body 320. Thecase main body 320 is closed by the lid 340 after the wound electrodeassembly 200 is placed therein. A joint portion 322 (see FIG. 1) betweenthe lid 340 and the case main body 320 is welded and sealed by, forexample, laser welding. Thus, in this example, the wound electrodeassembly 200 is positioned in the battery case 300 by the electrodeterminals 420 and 440 fixed to the lid 340 (i.e., the battery case 300).

<<Electrolyte Solution>>

Thereafter, an electrolyte solution is filled into the battery case 300through the filling port 350 provided in the lid 340. What is called anon-aqueous electrolyte solution, which does not use water as thesolvent, is used as the electrolyte solution. For example, theelectrolyte solution may be an electrolyte solution in which LiPF₆ iscontained at a concentration of about 1 mol/L in a mixed solvent ofethylene carbonate and diethyl carbonate (e.g., a mixed solvent with avolume ratio of about 1:1). After the electrolyte solution is filled,the battery case 300 is sealed by attaching (for example, by welding) ametal sealing cap 352 to the filling port 350. It should be noted thatthe electrolyte solution is not limited to the examples of theelectrolyte solution described herein. For example, any non-aqueouselectrolyte solution that has conventionally been used for lithium-ionsecondary batteries may be used as appropriate.

<<Pore>>

Here, the positive electrode active material layer 223 has tiny gaps225, which may be called voids, for example, between the positiveelectrode active material particles 610 and the particles of theconductive agent 620 (see FIG. 4). The tiny gaps in the positiveelectrode active material layer 223 can be impregnated with theelectrolyte solution (not shown). Also, the negative electrode activematerial layer 243 has tiny gaps 245, which may be called voids, forexample, between the particles of the negative electrode active materialparticles 710 (see FIG. 5). Herein, such gaps (or voids) 225 and 245 arereferred to as “pores” as appropriate. In addition, in the woundelectrode assembly 200, the uncoated portions 222 and 242 are wound in aspiral form at the respective sides along the winding axis WL, asillustrated in FIG. 2. The electrolyte solution can infiltrate throughthe gaps in the uncoated portions 222 and 242 at the respective sides252 and 254 along the winding axis WL. Thus, in the lithium-ionsecondary battery 100, the electrolyte solution is impregnatedthroughout the positive electrode active material layer 223 and thenegative electrode active material layer 243.

<<Gas Release Passage>>

In this example, the flat internal space of the battery case 300 isslightly wider than the wound electrode assembly 200 deformed in a flatshape. Gaps 310 and 312 are provided between the wound electrodeassembly 200 and the battery case 300 at the respective sides of thewound electrode assembly 200. Each of the gaps 310 and 312 serves as agas release passage. For example, when the temperature of thelithium-ion secondary battery 100 abnormally rises such as in the caseof overcharging, it is possible that the electrolyte solution may bedecomposed and gas may be generated abnormally. In this embodiment, theabnormally generated gas can move toward the safety vent 360 through thegaps 310 and 312 between the wound electrode assembly 200 and thebattery case 300, and is discharged out of the battery case 300 from thesafety vent 360.

In the lithium-ion secondary battery 100, the positive electrode currentcollector 221 and the negative electrode current collector 241 of thewound electrode assembly 200 are electrically connected to an externaldevice via the electrode terminals 420 and 440 penetrating through thebattery case 300. The operation of the lithium-ion secondary battery 100during charge and during discharge will be described in the following.

<<Operation During Charge>>

FIG. 7 schematically illustrates the state of the lithium-ion secondarybattery 100 during charge. During charge, the electrode terminals 420and 440 (see FIG. 1) of the lithium-ion secondary battery 100 areconnected to a charger 290, as illustrated in FIG. 7. By the working ofthe charger 290, lithium ions (Li) are released into the electrolytesolution 280 from the positive electrode active material in the positiveelectrode active material layer 223 during charge. In addition, electriccharge is released from the positive electrode active material layer223. The released electric charge is transferred through the conductiveagent (not shown) to the positive electrode current collector 221 andfurther transferred through the charger 290 to the negative electrodesheet 240. In the negative electrode sheet 240, electric charge isstored, and also the lithium ions (Li) in the electrolyte solution 280are absorbed and stored in the negative electrode active material withinthe negative electrode active material layer 243.

<<Operation During Discharge>>

FIG. 8 schematically illustrates the state of the lithium-ion secondarybattery 100 during discharge. During discharge, as illustrated in FIG.8, electric charge is transferred from the negative electrode sheet 240to the positive electrode sheet 220, and at the same time, the lithiumions stored in the negative electrode active material layer 243 arereleased into the electrolyte solution 280. In the positive electrode,the lithium ions in the electrolyte solution 280 are absorbed into thepositive electrode active material within the positive electrode activematerial layer 223.

Thus, in the charge and discharge of the lithium-ion secondary battery100, lithium ions are transferred back and forth between the positiveelectrode active material layer 223 and the negative electrode activematerial layer 243 via the electrolyte solution 280. In addition, duringcharge, electric charge is transferred from the positive electrodeactive material through the conductive agent to the positive electrodecurrent collector 221. On the other hand, during discharge, electriccharge is returned from the positive electrode current collector 221through the conductive agent to the positive electrode active material.

In the case of charge, when the transfer of lithium ions and thetransfer of electrons take place more smoothly, it is believed possibleto achieve more efficient and rapid charging. In the case of discharge,when the transfer of lithium ions and the transfer of electrons takeplace more smoothly, the resistance in the battery becomes lower and theamount of discharge becomes higher, so it is believed possible toimprove the output power of the battery.

<<Other Battery Constructions>>

It should be noted that the foregoing merely shows one example of thelithium-ion secondary battery. The lithium-ion secondary battery is notlimited to the foregoing embodiments. In addition, the electrode sheetin which an electrode mixture is coated on a metal foil may be used invarious other types of battery constructions. For example, cylindricalbatteries and laminate-type batteries are known as other types ofbattery constructions. The cylindrical battery is a battery in which awound electrode assembly is enclosed in a cylindrical battery case. Thelaminate-type battery is a battery in which positive electrode sheetsand negative electrode sheets are stacked on each other with separatorsinterposed therebetween.

Hereinbelow, a lithium-ion secondary battery according to one embodimentof the present invention will be described. Herein, the same componentsand portions having the same functions as those of the above-describedlithium-ion secondary battery 100 are denoted by the same referencesigns, and the drawings of the above-described lithium-ion secondarybattery 100 are referenced as necessary. It is also noted that thecomponents and portions of a lithium-ion secondary battery 100Aaccording to one embodiment of the present invention are denoted byadding the letter “A” to the corresponding reference signs asappropriate. In the present invention, the lithium-transition metaloxide used as the positive electrode active material is mainly designed.

FIG. 9 is a view illustrating the lithium-ion secondary battery 100Aaccording to one embodiment of the present invention. FIG. 10 is a viewillustrating a wound electrode assembly 200A of the lithium-ionsecondary battery 100A according to one embodiment of the presentinvention. FIG. 11 is a view illustrating the structure of a positiveelectrode active material layer 223A of the lithium-ion secondarybattery 100A. FIGS. 12A and 12B are electron micrographs of positiveelectrode active material particles 610A in the positive electrodeactive material layer 223A. FIG. 13 is a cross-sectional SEM image of apositive electrode active material particle 610A (secondary particle) inthe positive electrode active material layer 223A. FIG. 14 is aschematic view illustrating a primary particle 800 of the positiveelectrode active material particle 610A. FIG. 15 is a schematic viewillustrating a crystallite of the positive electrode active materialparticle 610A.

As illustrated in FIGS. 9 and 10, the lithium-ion secondary battery 100Ahas a positive electrode current collector 221A and a positive electrodeactive material layer 223A retained on the positive electrode currentcollector 221A. As illustrated in FIG. 11, the positive electrode activematerial layer 223A contains positive electrode active materialparticles 610A, a conductive agent 620A, and a binder 630A. Asillustrated in FIGS. 12A, 12B, and 13, for example, the positiveelectrode active material particles 610A each comprise a shell portion612 comprising primary particles 800 of a layered lithium-transitionmetal oxide, a hollow portion 614 formed inside the shell portion 612,and a through-hole (or through-holes) 616 penetrating through the shellportion 612. In addition, as illustrated in FIG. 14, the major axislength L1 of the primary particles 800 of the lithium-transition metaloxide is less than or equal to 0.8 μm.

In this lithium-ion secondary battery 100A, the positive electrodeactive material particles 610 each comprise the shell portion 612, thehollow portion 614, and the through-hole(s) 616, and the major axislength L1 of the primary particles 800 of the lithium-transition metaloxide is less than or equal to 0.8 μm, as described above. The presentinventor discovered that the use of such positive electrode activematerial particles 610A can improve the output power especially in a lowSOC region. Hereinbelow, such a lithium-ion secondary battery 100A willbe described in more detail.

<<Positive Electrode Active Material Particles 610>>

As illustrated in FIGS. 12A and 12B, the positive electrode activematerial particles 610A each comprise the shell portion 612 comprisingprimary particles of a layered lithium-transition metal oxide, thehollow portion 614 formed inside the shell portion 612, and thethrough-hole(s) 616 penetrating through the shell portion 612. It shouldbe noted that, in this description, the inner surface 612 a of the shellportion 612 does not include the portion(s) thereof that corresponds tothe through-hole(s) 616 of the positive electrode active materialparticle 610. In addition, the hollow portion 614 of the positiveelectrode active material particle 610 does not include thethrough-hole(s) 616. The structure of the positive electrode activematerial particle 610 that has the shell portion 612, the hollow portion614, and the through-hole(s) 616 in this way is herein referred to as“perforated hollow structure” as appropriate. In this lithium-ionsecondary battery 100A, the major axis length of the primary particlesof the lithium-transition metal oxide is less than or equal to 0.8 μm inaverage of the positive electrode active material layer 223A.

<<Primary Particles of Lithium-Transition Metal Oxide>>

Herein, the primary particle 800 of the lithium-transition metal oxideis such a particulate form as follows. When the positive electrodeactive material particles 610A are secondary particles, the primaryparticles 800 form the positive electrode active material particles 610Aas such secondary particles, and each primary particle 800 can beconsidered as an ultimate particle when judged from its apparentgeometric form. Each of the primary particles 800 is further anaggregate of crystallites of the lithium-transition metal oxide.

FIG. 12A is a cross-sectional SEM image showing a cross section of thepositive electrode active material layer 223A that is split by bendingit. FIG. 12B is an enlarged image of the positive electrode activematerial particles 610A in the SEM image of the just-mentioned crosssection. The primary particles 800 can be observed from a SEM image of across section obtained by bending and splitting the positive electrodeactive material layer 223A, for example, as illustrated in FIGS. 12A and12B. The primary particles 800 may also be observed from an electronmicrograph of the positive electrode active material particles 610A, aSEM image of particle surfaces of the positive electrode active materialparticles 610A, or the like. The SEM images of the split cross sectionof the positive electrode active material layer 223A, the electronmicrographs of the positive electrode active material particles 610A,the SEM images of particle surfaces of the positive electrode activematerial particles 610A, or the like can be obtained by, for example, aHitachi ultra-high resolution field emission scanning electronmicroscope S-5500.

<<Major Axis Length L1 of Primary Particles 800>>

The major axis length L1 of the primary particles 800 can be determinedbased on the positive electrode active material particles 610A observedfrom a SEM image of a cross section obtained by bending and splittingthe positive electrode active material layer 223A, for example, asillustrated in FIG. 12B. The major axis length L1 may also be determinedbased on an electron micrograph of the positive electrode activematerial particles 610A or a SEM image of particle surfaces of thepositive electrode active material particles 610A. In determining themajor axis length L1 of the primary particles 800 based on a SEM imageof particle surfaces of the positive electrode active material particles610A, as illustrated in FIG. 12B, it is desirable to identify primaryparticles 800 that are suitable for identifying the major axis length L1of the primary particles 800 from the SEM image of the particle surfacesof secondary particles, the positive electrode active material particles610A. More specifically, in the SEM image of the particle surfaces ofthe positive electrode active material particles 610A, which are thesecondary particles, a plurality of primary particles 800 arephotographed. These primary particles 800 should be ordered in order ofarea, and a plurality of primary particles 800 having large areas shouldbe extracted. This makes it possible to extract primary particles 800whose outer shape along substantially the longest major axis length L1are photographed from the SEM image of the particle surfaces. Then, asillustrated in FIG. 14, the length of the longest major axis isdetermined in the extracted primary particles 800, and the determinedlength is employed as the major axis length L1 of the primary particles800. Also, the length of the minor axis that is the shortest axisorthogonal to the major axis is employed as the minor axis length L2 ofthe primary particles 800.

Herein, when the major axis length L1 and the minor axis length L2 ofthe primary particles 800 are mentioned regarding the positive electrodeactive material particles 610A, they are evaluated by the arithmeticmean thereof in a plurality of primary particles 800 contained in asingle positive electrode active material particle 610A. For thepositive electrode active material layer 223A, they are evaluated by thearithmetic mean thereof in a plurality of positive electrode activematerial particles 610A contained in the positive electrode activematerial layer 223A.

<<Crystallite>>

In addition, as illustrated in FIG. 14, each of the primary particles800 is an aggregate of crystallites 810 of the lithium-transition metaloxide. Herein, the crystallite of the lithium-transition metal oxidemeans the largest collective unit that can be regarded as a singlecrystal of the lithium-transition metal oxide. In this embodiment, thecrystallite 810 of the lithium-transition metal oxide has a layeredstructure (layered rock-salt structure), as illustrated in FIG. 15, andit is believed that lithium ions move along the interlayer spaces withinthe positive electrode active material particles 610A during charge anddischarge. As illustrated in FIG. 15, the layers in the crystallite 810are stacked along the 003 plane orientation determined from an X-raydiffraction analysis using CuKα radiation.

<<Crystallite Size>>

Here, the present inventor measured the crystallite size of the positiveelectrode active material particles 610A along the 003 plane orientationdetermined from an X-ray diffraction analysis using CuKα radiation.Herein, the crystallite size D1 is obtained by the following equation.

D1=(0.9×λ)/((β×cos θ),

where D1, λ, β, and θ represent the following, respectively:D1: crystallite sizeλ: wavelength of X ray (CuKα) [Å]β: diffraction peak width originating from the crystallite [rad]θ: Bragg angle of diffractionHere, each of the positive electrode active material particles 610A has003 crystallites the 003 plane orientation determined from an X-raydiffraction analysis using CuKα radiation. Accordingly, the half width βof 17.9° to 19.9° is applied to the above equation as the Bragg angle ofdiffraction.

The present inventor discovered that the crystallite size (Å) of thepositive electrode active material particles 610A determined in theabove-described manner and the minor axis length L2 (μm) of the primaryparticles 800 correlate well each other. FIG. 16 illustrates therelationship between the minor axis length L2 (μm) of the primaryparticles 800 and the 003 plane crystallite size D1 (Å) of thelithium-transition metal oxide in the positive electrode active materialparticles 610A. The plots “∘” in FIG. 16 represent the minor axis lengthL2 of the primary particles 800 and the 003 plane crystallite size D1(Å) in a single positive electrode active material particle 610A. InFIG. 16, the horizontal axis represents the minor axis length L2 and thevertical axis represents the 003 plane crystallite size D1 (Å). Asindicated in FIG. 16, the longer the minor axis length L2 of the primaryparticles 800 is, the longer the 003 plane crystallite size D1 (Å)accordingly, so the relationship therebetween is approximately a linearcorrelation.

Here, the major axis length L1 of the primary particles 800 in thepositive electrode active material particle 610A is the length of theprimary particles 800 along one orientation orthogonal to theorientation of the minor axis length L2, as illustrated in FIG. 14.Accordingly, the present inventor believes that the major axis length L1of the primary particles 800 can approximately correlate with thecrystallite size along the orientation orthogonal to the orientation ofthe 003 plane crystallite size D1 (Å).

Here, the orientation of the 003 plane crystallite size D1 (Å) relatesto the orientation of the normal of the layers of the lithium-transitionmetal oxide, as illustrated in FIG. 15. The orientation orthogonal tothe orientation of the 003 plane crystallite size D1 (Å) is orthogonalto the orientation of the just-mentioned normal, and it indicates oneorientation along the interlayer spaces of the layeredlithium-transition metal oxide. It is believed that lithium ions movealong the interlayer spaces in the layered lithium-transition metaloxide. Accordingly, it is believed that the orientation orthogonal tothe orientation of the 003 plane crystallite size D1 (Å) correlates oneorientation L0 in which lithium ions generally diffuse, and the majoraxis length L1 of the primary particles 800.

The present inventor studied the relationship between the major axislength L1 of the primary particles 800 of the positive electrode activematerial particles 610A and the output power characteristics of thelithium-ion secondary battery 100A that was produced using the positiveelectrode active material particles 610A at a low charge level. FIG. 17illustrates the relationship between the major axis length L1 and theoutput power characteristics at a low charge level. Herein, the “outputpower characteristics in a charged state at −30° C. and at a SOC of 27%”were evaluated as the output power characteristics of the lithium-ionsecondary battery 100A at a low charge level. As illustrated in FIG. 17,when the major axis length L1 of the positive electrode active materialparticles 610A is smaller, the lithium-ion secondary battery 100A tendsto show better output power characteristics at a low charge level.According to the knowledge of the present inventor, it is desirable thatthe major axis length L1 of the positive electrode active materialparticles 610A be less than or equal to about 0.8 μm, more preferablyless than or equal to 0.75 μm.

Herein, as illustrated in FIGS. 9 to 11, the positive electrode activematerial particles 610A are contained in the positive electrode activematerial layer 223A of the wound electrode assembly 200A of thelithium-ion secondary battery 100A. The positive electrode activematerial layer 223A is formed by coating the positive electrode mixturecontaining the positive electrode active material particles 610A, thebinder, and so forth onto the positive electrode current collector 221A,drying the mixture, and pressure-rolling the resultant article. In suchan embodiment, the physical properties of the paste may be adverselyaffected when preparing the positive electrode active material layer223A if the major axis length L1 of the positive electrode activematerial particles 610A is too small, according to the knowledge of thepresent inventor. In addition, such an adverse effect may occur thatcracks can develop in the active material at the time of the pressingprocess. Taking these issues into account, it is desirable that themajor axis length L1 of the positive electrode active material particles610A used for the positive electrode active material layer 223A be aboutequal to or greater than 0.2 μm, more preferably equal to or greaterthan 0.4 μm.

In addition, a layered lithium-transition metal oxide is used as thepositive electrode active material particles 610A, and they have aperforated hollow structure, as mentioned previously.

Furthermore, the present inventor studied the relationship with theparticle void fraction for the positive electrode active materialparticles 610A having such a perforated hollow structure. Herein, theterm “particle void fraction” refers to the proportion of the hollowportions 614 in the positive electrode active material particles 610A.As a result, it was found that the particle void fraction shoulddesirably be equal to or greater than 23%.

In addition, the present inventor studied the relationship with thethickness of the shell portion 612 for the positive electrode activematerial particles 610A having such a perforated hollow structure.Herein, the thickness of the shell portion 612 at an arbitrary positionK in the inner surface of the shell portion 612 is defined by theminimum distance T(k) from an arbitrary position k to an outer surfaceof the shell portion 612 in an arbitrary cross section of the positiveelectrode active material layer 223A. As a result, it was founddesirable that in this lithium-ion secondary battery 100A, the thicknessT of the shell portion 612 be less than or equal to 2.2 μm in average inan arbitrary cross section of the positive electrode active materiallayer 223A.

<<Proportion of Hollow Portion 614: Particle Void Fraction>>

Herein, the proportion of the hollow portions 614 in the positiveelectrode active material particles 610A can be determined, for example,based on a cross-sectional SEM image of the positive electrode activematerial layer 223A. As for the cross-sectional SEM image of thepositive electrode active material layer 223A, the shell portion 612,the hollow portion 614, and the through-hole 616 of the positiveelectrode active material particle 610A can be distinguished from eachother by difference in color tone or contrast in the cross-sectional SEMimage of the positive electrode active material layer 223A, asillustrated in FIG. 13.

Based on an arbitrary cross-sectional SEM image of the positiveelectrode active material layer 223A, the ratio (A/B) of the area Aoccupied by the hollow portion 614A of the positive electrode activematerial particle 610A to the apparent cross-sectional area B occupiedby the positive electrode active material particle 610A in thecross-sectional SEM image is obtained. Here, the apparentcross-sectional area B occupied by the positive electrode activematerial particle 610A is the cross-sectional area occupied by the shellportion 612, the hollow portion 614 and the through-hole(s) 616 of thepositive electrode active material particle 610A.

Furthermore, for a plurality of arbitrary cross-sectional SEM images ofthe positive electrode active material layer 223A, the mean value of theabove-described ratios (A/B) is obtained. The greater the number of thecross-sectional SEM images for obtaining the area ratio (A/B) in such across-sectional SEM image, the more the mean value of the ratios (A/B)in the positive electrode active material layer 223A converges. Theproportion of the area occupied by the hollow portions 614 in theapparent cross-sectional area of the positive electrode active materialparticles 610A in average of the positive electrode active materiallayer 223A can be approximately obtained by the mean value of the ratios(A/B). The proportion of the cross-sectional area occupied by the hollowportions 614 in the apparent cross-sectional area of the positiveelectrode active material particles 610A in average of the positiveelectrode active material layer 223A is referred to as “particle voidfraction” as appropriate.

<<Thickness T of Shell Portion 612>>

In this case, the minimum distance T(k) is obtained for a plurality ofpositions in the inner surface 612 a of the shell portion 612. Then, itis desirable to calculate the mean value of the minimum distances T(k)for the plurality of positions in the inner surface 612 a of the shellportion 612. In this case, the greater the number of the positions atwhich the minimum distance T(k) is obtained in the inner surface 612 aof the shell portion 612, the more the thickness T of the shell portion612 converges to the mean value, which reflects the thickness of theshell portion 612 more accurately. With this method, the thickness ofthe shell portion 612 is determined uniquely for an arbitrary position kin the inner surface 612 a of the shell portion 612. As a result, evenwhen the cross-sectional shape of the shell portion 612 is irregular,the thickness T of the shell portion 612 can be generally defineduniquely over the whole of the positive electrode active materialparticles 610A.

As illustrated in FIG. 13, the positive electrode active materialparticles 610A each comprise the shell portion 612, the hollow portion614, and the through-hole(s) 616, and the inside of the shell portion612 (i.e., the hollow portion 614) and the outside of the shell portion612 communicate with each other through the through-hole(s) 616. Such apositive electrode active material particle 610 is referred to as aperforated hollow structure as appropriate. The positive electrodeactive material particle 610A is such that, for example, the proportionof the hollow portion 614 is equal to or greater than 23% of theapparent cross-sectional area of the positive electrode active materialparticle 610A, which means that it has a wide hollow portion 614.Moreover, the positive electrode active material particle 610A has thethrough-hole(s) 616 penetrating through the shell portion 612.Therefore, the electrolyte solution 280 (see FIGS. 7 and 8) also entersthe inside of the shell portion 612 (i.e., the hollow portion 614)through the through-holes 616. In this positive electrode activematerial particle 610A, the hollow portion 614 is wide. As a result, theelectrolyte solution 280 containing lithium ions sufficiently exists notonly in the outside part of the shell portion 612 but also in the insideof the shell portion 612 (i.e., the hollow portion 614). Moreover, inthis lithium-ion secondary battery 100A, the thickness T of the shellportion 612 of the positive electrode active material particle 610A isthin, less than or equal to 2.2 μm in average in an arbitrary crosssection of the positive electrode active material layer 223A.

According to the knowledge of the present inventor, the thinner thethickness T of the shell portion 612 of the positive electrode activematerial particle 610A is, the more easily the lithium ions are releasedfrom the inside of the shell portion 612 of the positive electrodeactive material particle 610A during charge, and the more easily thelithium ions are absorbed into the inside of the shell portion 612 ofthe positive electrode active material particle 610A during discharge.

This lithium-ion secondary battery 100A is such that the major axislength L1 of the primary particles 800 of the positive electrode activematerial particles 610A is less than or equal to 0.8 μm in average ofthe positive electrode active material layer 223A. When the major axislength L1 of the primary particles 800 of the positive electrode activematerial particles 610A is less than or equal to 0.8 μm in this way, thedistance in the direction in which lithium ions diffuse is short in theprimary particles 800 of the positive electrode active materialparticles 610A. Therefore, it is believed that the positive electrodeactive material particles 610A have good lithium ion diffusioncapability.

In addition, it is preferable that the proportion of the hollow portions614 in the positive electrode active material particles 610A be equal toor greater than 23%, and the positive electrode active materialparticles 610A have the through-holes 616 penetrating through the shellportion 612, and moreover, the thickness T of the shell portion 612 ofthe positive electrode active material particles 610A be very thin (lessthan or equal to 2.2 μm herein). Such positive electrode active materialparticles 610A enable lithium ions to diffuse into the inner portion ofthe shell portion 612 (i.e., the inside of the active material) morequickly.

When this is the case, lithium ions are released easily from the insideof the shell portion 612 of the positive electrode active materialparticle 610A during charge, and lithium ions are absorbed easily intothe inside of the shell portion 612 of the positive electrode activematerial particle 610A during discharge. Thus, the positive electrodeactive material particles 610A are such that the major axis length L1 ofthe primary particles 800 is short, they show good lithium ion diffusioncapability, and moreover, the thickness of the shell portion 612 isthin. This contributes to smooth lithium ion release from and absorptioninto the inside of the shell portion 612 and moreover the primaryparticles 800 during charge and discharge of the lithium-ion secondarybattery 100A. This enables to increase the amount of the lithium ionsthat are released and absorbed per unit weight of the positive electrodeactive material particles 610A and also to reduce the resistance at thetime when the positive electrode active material particles 610A releaseand absorb lithium ions.

Generally, the lithium-ion secondary battery tends to show a decrease inthe output power when the charge level is low (for example, when the SOCis less than or equal to 30%). However, this lithium-ion secondarybattery 100A can maintain required output power even at a low chargelevel because the lithium ion diffusion capability in the positiveelectrode active material particles 610A is high even when the chargelevel is low.

The positive electrode active material particles 610A each comprise theshell portion 612, the hollow portion 614, and the through-hole(s) 616,as described above. The major axis length L1 of the primary particles800 is less than or equal to 0.8 μm. More preferably, the hollow portion614 is wide, and the shell portion 612 is thin. The positive electrodeactive material particles 610A of this type have not been commonly knownin the past. For example, the proportion of the hollow portion 614 isequal to or greater than 23% of the apparent cross-sectional area of thepositive electrode active material particle 610A, which means that suchpositive electrode active material particles 610A are clearlydistinguished from mere sintered materials.

<<Manufacturing Method of Positive Electrode Active Material Particles610>>

The following describes a suitable method of manufacturing the positiveelectrode active material particles 610A that can obtain such positiveelectrode active material particles 610A stably.

The method of manufacturing the positive electrode active materialparticles 610A includes, for example, a source hydroxide producing step,a mixing step, and a sintering step. The source hydroxide producing stepis a step of supplying ammonium ions to an aqueous solution of atransition metal compound to precipitate particles of the transitionmetal hydroxide from the aqueous solution. Here, the aqueous solutioncontains at least one transition metal element that constitutes thelithium-transition metal oxide.

Herein, it is desirable that the source hydroxide producing step includea nucleation stage of precipitating a transition metal hydroxide fromthe aqueous solution and a particle growth stage of growing thetransition metal hydroxide in a condition in which the pH of the aqueoussolution is lower than that at the nucleation stage.

The mixing step is a step of mixing a lithium compound and thetransition metal hydroxide to prepare an unsintered mixture. Thesintering step is a step of sintering the mixture to obtain the positiveelectrode active material particles 610A. More preferably, it isdesirable to pulverize the sintered material after the sintering and tosieve and classify the material.

Hereinbelow, the method of manufacturing the positive electrode activematerial particles 610A will be described in more detail.

The perforated hollow active material particles disclosed herein can bemanufactured by, for example, the method including precipitating ahydroxide of at least one transition metal element contained in thelithium-transition metal oxide that constitutes the active materialparticles (preferably all the metal elements contained in thelithium-transition metal oxide other than lithium) from an aqueoussolution containing the transition metal element(s) under an appropriatecondition, then mixing the transition metal hydroxide with a lithiumcompound, and sintering the mixture. Hereinbelow, one embodiment of themethod of manufacturing the active material particles will be describedin detail, taking a case of manufacturing perforated hollow activematerial particles comprising a layered LiNiCoMn oxide as an example.However, the applications of this manufacturing method are not limitedto manufacturing the perforated hollow active material particles havingsuch a composition. In addition, unless otherwise stated, the method ofmanufacturing the positive electrode active material particles 610Aaccording to the present invention is not limited to such amanufacturing method.

<<Source Hydroxide Producing Step>>

The method of manufacturing active material particles disclosed hereinincludes a step of supplying ammonium ions (NH⁴⁺) to an aqueous solutionof a transition metal compound to precipitate particles of a transitionmetal hydroxide from the aqueous solution (source hydroxide producingstep). The solvent (aqueous solvent) that constitutes theabove-mentioned aqueous solution is typically water, but may be a mixedsolvent containing water as its main component. A suitable example ofthe solvent that constitutes the mixed solvent other than water is anorganic solvent that can mix with water uniformly (such as a loweralcohol). Depending on the composition of the lithium-transition metaloxide that constitutes the active material particles that are the targetobject to be manufactured, the above-described aqueous solution of atransition metal compound (hereinafter also referred to as “transitionmetal solution”) contains at least one of (preferably all of) thetransition metal elements (herein, Ni, Co, and Mn) that constitute thelithium-transition metal oxide. For example, it is possible to use atransition metal solution containing one compound, or two or morecompounds, that can supply Ni ions, Co ions, and Mn ions into theaqueous solvent. As the compounds that serve as the source of thesemetallic ions, it is possible to employ sulfates, nitrates, chlorides,or the like of those metals appropriately. For example, it is possibleto use a transition metal solution having the composition in whichnickel sulfate, cobalt sulfate, and manganese sulfate are dissolved inan aqueous solvent (preferably water).

The NH₄ ⁺ may be supplied to the transition metal solution, for example,in the form of aqueous solution (typically water solution) containingNH₄ ⁺, or may be supplied by blowing ammonia gas directly into thetransition metal solution. These supplying methods may be combined witheach other. The aqueous solution containing NH₄ ⁺ can be prepared by,for example, dissolving a compound that can be the source of NH₄ ⁺ (suchas ammonium hydroxide, ammonium nitrate, or ammonia gas) into an aqueoussolvent. In the present embodiment, NH₄ ⁺ is supplied in the form ofammonium hydroxide aqueous solution (i.e., ammonia water).

<<Nucleation Stage>>

The source hydroxide producing step can include a stage (nucleationstage) of precipitating a transition metal hydroxide from the transitionmetal solution under the conditions of equal to or higher than pH 12(typically from pH 12 to pH 14, for example, from pH 12.2 to 13) and aNH₄ ⁺ concentration of less than or equal to 25 g/L (typically from 3g/L to 25 g/L). The just-mentioned pH and the NH₄ ⁺ concentration may beadjusted by appropriately balancing the amounts of the ammonia water andthe alkaline agent (the compound that can cause the solution to becomealkaline) that are used. Examples of the alkaline agent include sodiumhydroxide and potassium hydroxide, which can be typically used in theform of aqueous solution. The present embodiment employs a sodiumhydroxide aqueous solution. It should be noted that the pH valuesmentioned in the present description are the pH values determined at aliquid temperature of 25° C. as the reference.

<<Particle Growth Stage>>

The source hydroxide producing step can further include a stage(particle growth stage) of growing the nuclei of the transition metalhydroxide (typically in a particulate form) that have been precipitatedin the nucleation stage at less than pH 12 (typically from pH 10 to lessthan pH 12, preferably from pH 10 to pH 11.8, for example, from pH 11 topH 11.8) and a NH₄ ⁺ concentration of equal to or higher than 1 g/L,preferably 3 g/L, (typically from 3 g/L to 25 g/L). Normally, it isappropriate to set the pH during the particle growth stage to be lowerby 0.1 or greater (typically 0.3 or greater, preferably 0.5 or greater,for example, from about 0.5 to about 1.5) than the pH during thenucleation stage.

The just-mentioned pH and the NH₄ ⁺ concentration can be adjusted in thesame manner as in the nucleation stage. This particle growth stageallows the rate of precipitation of the transition metal hydroxide (acomposite hydroxide containing Ni, Co, and Mn herein) to be quicker bysatisfying the pH and the NH₄ ⁺ concentration described above,preferably by setting the NH₄ ⁺ concentration to be within the range ofless than or equal to 15 g/L (e.g., from 1 g/L to 15 g/L, typically from3 g/L to 15 g/L), more preferably less than or equal to 10 g/L (e.g.,from 1 g/L to 10 g/L, typically from 3 g/L to 10 g/L) in theabove-described pH range. Thus, it becomes possible to produce thesource hydroxide particles that are suitable for the formation of theperforated hollow active material particles disclosed herein (in otherwords, the source hydroxide particles that can easily form a sinteredmaterial having a perforated hollow structure).

The NH₄ ⁺ concentration may be set to less than or equal to 7 g/L (e.g.,from 1 g/L to 7 g/L, more preferably from 3 g/L to 7 g/L). The NH₄ ⁺concentration at the particle growth stage may be, for example,approximately the same level as the NH₄ ⁺ concentration at thenucleation stage, or may be lower than the NH₄ ⁺ concentration at thenucleation stage. The rate of precipitation of the transition metalhydroxide can be found by, for example, investigating the change of thetotal mole number (total ion concentration) of the transition metal ionscontained in the liquid phase of the reaction solution with respect tothe total mole number of the transition metal ions contained in thetransition metal solution supplied to the reaction solution.

It is preferable that the temperature of the reaction solution becontrolled to approximately a constant temperature (for example, ±1° C.of a predetermined temperature) within the range of from about 30° C. toabout 60° C. in both the nucleation stage and the particle growth stage.It is possible to set the temperature of the reaction solution to thesame level both in the nucleation stage and the particle growth stage.In addition, it is preferable that the atmosphere of the reactionsolution and in the reaction chamber be kept to be a non-oxidizingatmosphere throughout the nucleation stage and the particle growthstage. In addition, the total mole number (total ion concentration) ofNi ions, Co ions, and Mn ions contained in the reaction solution may beset to be from about 0.5 mol/L to about 2.5 mol/L throughout thenucleation stage and the particle growth stage, preferably from about1.0 mol/L to 2.2 mol/L. It is desirable that the transition metalsolution be supplemented (typically continuously supplied) according tothe rate of precipitation of the transition metal hydroxide so that sucha total ion concentration can be maintained. It is preferable that theamounts of the Ni ions, the Co ions, and the Mn ions contained in thereaction solution may be set to the quality ratio corresponding to thecomposition of the active material particles that are the targetmaterial (i.e., the mole ratio of Ni, Co, and Mn in the LiNiCoMn oxidethat constitutes the active material particles).

<<Mixing Step>>

In the present embodiment, the transition metal hydroxide particles(composite hydroxide particles containing Ni, Co, and Mn herein) areseparated from the reaction solution, then washed, and dried. Then, thetransition metal hydroxide particles and a lithium compound are mixed ata desired quantity ratio to prepare an unsintered mixture (mixing step).In this mixing step, typically, the Li compound and the transition metalhydroxide particles are mixed at a quantity ratio corresponding to thecomposition of the active material particles that are the targetmaterial (i.e., the mole ratio of Li, Ni. Co, and Mn in the LiNiCoMnoxide that constitutes the active material particles). Preferableexamples of the lithium compound include Li compounds that can be meltedand turned into an oxide by heating, such as lithium carbonate andlithium hydroxide.

<<Sintering Step>>

Then, the mixture is sintered to obtain active material particles(sintering step). This sintering step is typically performed in anoxidizing atmosphere (for example, in the air (i.e., in the airatmosphere)). The sintering temperature in this sintering step may beset to, for example, from 700° C. to 1100° C. It is preferable that themaximum sintering temperature be 800° C. or higher (preferably from 800°C. to 1100° C., for example, from 800° C. to 1050° C.). With the maximumsintering temperature in these ranges, the sintering reaction of theprimary particles of the lithium-transition metal oxide (preferablyNi-containing Li oxide, LiNiCoMn oxide herein) can be allowed to proceedappropriately.

One preferable embodiment includes a first sintering stage of sinteringthe mixture at a temperature T1 of from 700° C. to 900° C. (that is,700° C.≦T1≦900° C., for example, 700° C.≦T1≦800° C., typically 700°C.≦T1≦800° C.) and a second sintering stage of sintering the resultantarticle that has undergone the first sintering stage at a temperature T2of from 800° C. to 1100° C. (that is, 800° C.≦T2≦1100° C., for example,800° C.≦T2≦1050° C.). This enables to form the active material particleswith the perforated hollow structure efficiently. It is preferable thatT1 and T2 be set so that T1<T2.

The first sintering stage and the second sintering stage may be carriedout consecutively (for example, it is possible that, after the mixtureis kept at the first sintering temperature T1, the temperature iscontinuously elevated to the second sintering temperature T2 and kept atthe temperature T2). Alternatively, it is also possible that, afterhaving been kept at the first sintering temperature T1, the material iscooled (for example, cooled to room temperature), then pulverized, andsieve-classified as necessary, and thereafter supplied to the secondsintering stage.

In the technique disclosed herein, the first sintering stage can beunderstood as a stage during which sintering is carried out at thetemperature T1 that is within a temperature range at which the sinteringreaction for the target lithium-transition metal oxide proceeds, andlower than or equal to the melting point, and is lower than thetemperature of the second sintering stage. The second sintering stagecan be understood as a stage during which sintering is carried out atthe temperature T2 that is in a temperature range at which the sinteringreaction for the target lithium-transition metal oxide proceeds, andlower than or equal to the melting point, but is higher than thetemperature of the first sintering stage. It is preferable to provide atemperature difference of equal to or greater than 50° C. or more(typically equal to or greater than 100° C., for example, equal to orgreater than 150° C.) between T1 and T2.

Thus, the method of manufacturing the positive electrode active materialparticles 610A includes the source hydroxide producing step, the mixingstep, and the sintering step. Herein, it is desirable that the positiveelectrode active material particles 610A can be stably obtained in whichthe proportion of the hollow portion 614 in the apparent cross-sectionalarea of the positive electrode active material particle 610A is equal toor greater than 23%, and the thickness T of the shell portion 612 of thepositive electrode active material particle 610A is thin, less than orequal to 2.2 μm. The following describes a method of manufacturingpositive electrode active material particles 610A, which enables toobtain the positive electrode active material particles 610A such asdescribed above more stably.

In order to obtain the positive electrode active material particles 610Amore stably, it is desirable to appropriately adjust the pH or the NH⁴⁺concentration at the stage of precipitating the transition metalhydroxide from the transition metal solution (i.e., the nucleationstage) and the pH or the NH⁴⁺ concentration at the stage (particlegrowth stage) of growing the nuclei of the transition metal hydroxidethat have been precipitated at the nucleation stage.

In such a transition metal solution, the following equilibrium reactionis taking place, for example.

[Chemical formula 1]

(M1)²⁺+6(NH₃)

[M1(NH₃)₆]²⁺  Reaction 1

[Chemical formula 2]

(M1)²⁺+2OH⁻

M1(OH)₂  Reaction 2

Here, M1 represents the transition metals contained in the transitionmetal solution, which include Ni in this embodiment.

That is, in the equilibrium reaction of Reaction 1, the reaction betweenthe transition metal (M1) in the transition metal solution, the ammonia(NH³) supplied to the transition metal solution, and the compound[M1(NH₃)₆]²⁺ of the transition metal (M1) and ammonia (NH³) is inequilibrium. In the equilibrium reaction of Reaction 2, the reactionbetween the transition metal (M1) in the transition metal solution, thehydroxide ions (OH⁻) supplied to the transition metal solution, and thetransition metal hydroxide [M1(OH₂] is in equilibrium.

In this case, when the pH in the transition metal solution decreases,the transition metal hydroxide (M1(OH)₂) tends to precipitate easily bythe equilibrium reaction of Reaction 2. At this time, the transitionmetal hydroxide (M1(OH)₂) is allowed to precipitate easily by keepingthe amount of ammonia in the transition metal solution small, proceedingthe equilibrium expression to the left side, and increasing thetransition metal ions (M1)2₊ in the transition metal solution. Thus, thetransition metal hydroxide (M1(OH)₂) is allowed to precipitate easily bykeeping the amount of ammonia in the transition metal solution small anddecreasing the pH in the transition metal solution.

For example, at the nucleation stage, the pH should be kept to a certaindegree while keeping the solubility of the ammonia (NH₃) in thetransition metal solution to be low. This makes it possible to controlthe rate of precipitation of the transition metal hydroxide (M1(OH)₂)appropriately. This allows the density of the inner portions of theparticles of the transition metal hydroxide, which serve as theprecursor, to be low. In addition, at the particle growth stage, the pHshould be decreased while keeping the solubility of the ammonia (NH₃) inthe transition metal solution to be low. This accelerates the rate ofprecipitation of the transition metal hydroxide (M1(OH)₂) at thenucleation stage. As a result, the density of the particles of thetransition metal hydroxide, which serve as the precursor, is made highernear the outer surfaces thereof than the density of the inner portionsof the particles of the transition metal hydroxide.

Thus, it is possible to make the density of the transition metalhydroxide lower in the inside of the particles and higher near the outersurfaces thereof by appropriately adjusting the pH and the ammoniaconcentration (ammonium ion concentration) of the transition metalsolution at the nucleation stage and at the particle growth stage.

Here, it is desirable that, for example, the pH of the transition metalsolution be from 12 to 13 at the nucleation stage, and that the pH ofthe aqueous solution be from 11 to less than 12 at the particle growthstage. At this time, it is desirable that the pH of the transition metalsolution at the nucleation stage be decreased by equal to or greaterthan 0.1, more preferably by equal to or greater than 0.2, from that atthe particle growth stage. In addition, it is desirable that the ammoniaconcentration (ammonium ion concentration) at the particle growth stagebe kept low, from 3 g/L to 10 g/L. This ensures that the rate ofprecipitation of the transition metal hydroxide (M1(OH)₂) becomesquicker at the particle growth stage than at the nucleation stage.Moreover, the density of the particles of the transition metal hydroxideis made higher near the outer surfaces thereof than density of the innerportions of the particles of the transition metal hydroxide morereliably.

It should be noted that the hollow portion 614 of the positive electrodeactive material particle 610A can be made larger by taking a necessarytime at the nucleation stage. In addition, the shell portion 612 of thepositive electrode active material particle 610A can be made thinner byincreasing the rate of precipitation of the transition metal hydroxideat the particle growth stage and also shortening the time of theparticle growth stage.

Additionally, in this case, it is desirable to keep the amount ofammonia in the transition metal solution small. For example, it isdesirable that the ammonium ion concentration in the transition metalsolution at the nucleation stage be less than or equal to 20 g/L, andthat the ammonium ion concentration in the transition metal solution atthe particle growth stage be less than or equal to 10 g/L. Thus, it ispossible to maintain the ion concentration of the transition metalcontained in the transition metal solution to be the necessary amount,by keeping the ammonium ion concentration of the transition metalsolution at the nucleation stage and at the particle growth stage. Inthis case, it is undesirable if the amount of ammonia is too small inthe transition metal solution. It is desirable that the ammonium ionconcentration in the transition metal solution at the nucleation stageand at the particle growth stage be, for example, equal to or greaterthan 3 g/L.

In the mixing step, a lithium compound and the transition metalhydroxide are mixed to prepare an unsintered mixture. In the sinteringstep, the mixture is sintered to obtain positive electrode activematerial particles 610A. Here, the particle of the transition metalhydroxide, which serves as the precursor of the positive electrodeactive material particle 610A, has a low density in the inner portionthereof and a high density near the outer surface thereof. As a result,the density of the particles of the transition metal hydroxide, whichserve as the precursor, is made higher near the outer surfaces thereofthan the density of the inner portions of the particles of thetransition metal hydroxide. This enables to form a shell portion 612 ofthe positive electrode active material particle 610A and also to form alarge hollow portion 614. Moreover, when crystals are grown at the timeof sintering, a through-hole 616 penetrating through the shell portion612 is formed in a portion of the shell portion 612. Thus, the positiveelectrode active material particles 610 each having the shell portion612, the hollow portion 614, and the through-hole(s) 616 are formed, asillustrated in FIG. 13. Preferably, it is desirable that the sinteredmaterial be pulverized after the sintering step, and thensieve-classification is performed, to adjust the particle size of thepositive electrode active material particles 610A.

The thus produced positive electrode active material particles 610A eachcomprise the thin shell portion 612, the wide hollow portion 614, andthe through-hole(s) 616 penetrating through the shell portion 612 andspatially connecting the hollow portion 614 and the outside of the shellportion 612 of the positive electrode active material particle 610A witheach other. In one preferable embodiment of such positive electrodeactive material particles 610A, it is possible that the BET specificsurface area of the positive electrode active material particles 610A beset to about 0.3 m²/g to about 2.2 m²/g. More preferably, the BETspecific surface area of the positive electrode active materialparticles 610A may be set to equal to or greater than about 0.5 m²/g,still more preferably equal to or greater than about 0.8 m²/g. The BETspecific surface area of the positive electrode active materialparticles 610A may also be set to, for example, less than or equal toabout 1.9 m²/g, more preferably less than or equal to 1.5 m²/g.

In such positive electrode active material particles 610A, the densityof the shell portion 612 is high since the source hydroxide producingstep includes the nucleation stage and the particle growth stage.Therefore, it is possible to obtain the positive electrode activematerial particles 610A that are harder and more morphologically stablethan those obtained by other manufacturing methods (such as a spraysintering method (also referred to as a spray drying method)).

Such positive electrode active material particles 610A have an averagehardness of 0.5 MPa or higher, as determined by a dynamic hardness testthat is carried out, for example, using a 50 μm-diameter flat diamondindenter under the condition of a loading rate of from 0.5 mN/sec. to 3mN/sec.

In another preferable embodiment of the active material particlesdisclosed herein, the average hardness of the positive electrode activematerial particles 610A is equal to or greater than about 0.5 MPa. Theterm average hardness herein refers to the values determined by adynamic microhardness test that is carried out using a 50 μm-diameterflat diamond indenter under the condition of a loading rate of from 0.5mN/sec. to 3 mN/sec. For this kind of dynamic microhardness test, it ispossible to use, for example, a microhardness tester MCT-W500 made byShimadzu Corp.

Thus, the positive electrode active material particles 610A have ahollow structure, as illustrated in FIG. 13, and high average hardness(in other words, high shape retention capability). Such positiveelectrode active material particles 610A can offer a battery that showshigher performance stably. Therefore, the positive electrode activematerial particles 610A are highly suitable for constructing, forexample, a lithium-ion secondary battery that has low internalresistance (in other words, good output power characteristics) and showslow resistance increase even with charge-discharge cycles (especiallywith charge-discharge cycles involving discharge at high rate).

<<Lithium-Transition Metal Oxide that Constitutes the Positive ElectrodeActive Material Particles 610A>>

In manufacturing the positive electrode active material particles 610A,it is particularly desirable that the transition metal solution containnickel. In the case where the transition metal solution contains nickel,particles of the transition metal hydroxide are produced in the form ofsecondary particles, in which a plurality of fine primary particles in arice grain-like shape are aggregated, when the transition metal solutionprecipitates at the nucleation stage and the particle growth stage.Within the temperature range in sintering, crystals grow while almostmaintaining the shape of the primary particles of the transition metalhydroxide.

It should be noted that, in the case where the transition metal solutiondoes not contain nickel but contains cobalt and consequently particlesof lithium cobalt oxide (LiCoO₂) are produced by sintering, the shape ofthe primary particles cannot be maintained, and the entire particle issintered. When this is the case, it is difficult to obtain such positiveelectrode active material particles 610A each having a large hollowportion 614 (see FIG. 13) as described above.

Thus, in order to manufacture the positive electrode active materialparticles 610A stably, the lithium-transition metal oxide may preferablybe a layered compound containing nickel as a constituent elementthereof. When nickel is contained, it is possible to form particles ofthe transition metal hydroxide (precursor particles) in which thedensity of the inner portions thereof is low and the density of theportions thereof near the outer surfaces is high. Using such precursorparticles in which the density of the inner portions thereof is low andthe density of the portions thereof near the outer surfaces is high,crystals can be grown in the sintering step while almost maintaining theshape of the primary particles. This makes it possible to manufacturethe positive electrode active material particles 610A each comprise theshell portion 612, the hollow portion 614, and the through-hole(s) 616(see FIG. 13).

In this case, it is desirable that proportion of nickel (compositionratio) is about equal to or greater than 0.1%, more preferably equal toor greater than 0.25%, in the transition metals contained in thepositive electrode active material particles 610A.

In addition, the lithium-transition metal oxide may be a layeredcompound containing nickel, cobalt, and manganese as its constituentelements. For example, it is desirable that the lithium-transition metaloxide be a layered compound contained asLi_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂. In the formula, 0≦x≦0.2,0.1<y<0.9, 0.1<z<0.4, M is an additive, and 0≦γ≦0.01. For example, it isdesirable that M be at least one additive selected from the groupconsisting of Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F. Sucha lithium-transition metal oxide forms a layered compound, which canretain lithium ions in the interlayer spaces. Moreover, it isparticularly suitable for manufacturing the positive electrode activematerial particles 610A each comprising the shell portion 612, thehollow portion 614, and the through-hole(s) 616.

Thereby, it is possible to stably obtain the positive electrode activematerial particles 610A in which the proportion of the hollow portion614 in the apparent cross-sectional area of the positive electrodeactive material particle 610A is equal to or greater than 23%, and thethickness T of the shell portion 612 of the positive electrode activematerial particle 610A is thin, less than or equal to 2.2 μm.

As described previously, the lithium-ion secondary battery 100A has thepositive electrode current collector 221A (current collector) and theporous positive electrode active material layer 223A retained on thepositive electrode current collector 221A, as illustrated in FIGS. 1through 3. As illustrated in FIG. 4, the positive electrode activematerial layer 223A contains the positive electrode active materialparticles 610A, the conductive agent 620A, and the binder 630A. In thisembodiment, as illustrated in FIG. 13, the positive electrode activematerial particles 610A each comprise the shell portion 612 comprising alithium-transition metal oxide, the hollow portion 614 formed inside theshell portion 612, and the through-hole(s) 616 penetrating through theshell portion 612.

In this lithium-ion secondary battery 100A, the proportion of the hollowportion 614 is equal to or greater than 23% of the apparentcross-sectional area of the positive electrode active material particle610A (see FIG. 13), in average of the positive electrode active materiallayer 223A. In addition, the thickness of the shell portion 612 at anarbitrary position in the inner surface 612 a of the shell portion 612is defined by the minimum distance T(k) from an arbitrary position k toan outer surface of the shell portion 612 in an arbitrary cross sectionof the positive electrode active material layer 223A. In this case, thethickness of the shell portion 612 is less than or equal to 2.2 μm inaverage in an arbitrary cross section of the positive electrode activematerial layer 223A.

In this lithium-ion secondary battery 100A, the proportion of the hollowportion 614 is equal to or greater than 23% of the apparentcross-sectional area of the positive electrode active material particle610A (see FIG. 13), in average of the positive electrode active materiallayer 223A, which means that the hollow portion 614 is large. In thislithium-ion secondary battery 100A, the electrolyte solution 280 (seeFIGS. 7 and 8) can be impregnated sufficiently into the hollow portion614 of each of the positive electrode active material particles 610A inpositive electrode active material layer 223A. Moreover, in thislithium-ion secondary battery 100A, the thickness of the shell portion612 is less than or equal to 2.2 μm, in average in an arbitrary crosssection of the positive electrode active material layer 223A, whichmeans that the shell portion 612 of the positive electrode activematerial particle 610A is thin. As a result, the positive electrodeactive material particles 610A allow lithium ions to quickly diffuseinto the inner portion of the shell portion 612 (i.e., the inside of theactive material). Therefore, the lithium-ion secondary battery 100A canproduce high output power even at a low charge level.

In this case, it is desirable that the thickness of the shell portion612 be equal to or greater than 0.05 μm, more preferably equal to orgreater than 0.1 μm, in average of the positive electrode activematerial layer 223A. When the thickness of the shell portion 612 beequal to or greater than 0.05 μm, more preferably equal to or greaterthan 0.1 μm, a required mechanical strength is provided for the positiveelectrode active material particles 610A. The positive electrode activematerial particles 610A undergo expansion and shrinkage as the releaseand absorption of lithium ions are repeated. Sufficient strength isensured for expansion and shrinkage Therefore, the durability of thepositive electrode active material particles 610A is improved, and theperformance of the lithium-ion secondary battery 100A can be made stableover time.

It is also desirable that the through-hole(s) 616 have an aperture widthof equal to or greater than 0.01 μm in average of the positive electrodeactive material layer 223A. Herein, the aperture width of a through-hole616 is the length across the narrowest portion of the path in which thethrough-hole 616 extends from the outside of the positive electrodeactive material particle 610A to the hollow portion 614. When theaperture width of the through-holes 616 is 0.01 μm in average, theelectrolyte solution 280 (see FIGS. 7 and 8) can sufficiently enter thehollow portion 614 from outside through the through-holes 616. As aresult, the advantageous effect of improving the battery performance ofthe lithium-ion secondary battery 100A can be obtained moreappropriately.

Generally, the thin shell portion 612, the wide hollow portion 614, andthe through-holes 616 with wide aperture width such as in the positiveelectrode active material particles 610A cannot be achieved by othermanufacturing methods (such as a spray sintering method (also referredto as a spray drying method)).

The mean value of the above-mentioned aperture width (average aperturesize) can be obtained by, for example, taking at least 10 samples of thepositive electrode active material particles 610A, finding the aperturewidths of a portion or all of the through holes 616 for each sample ofthe positive electrode active material particles 610A, and obtaining thearithmetic mean value thereof. It is sufficient that the through-hole(s)616 should be suitable for the electrolyte solution 280 to impregnateinto the hollow portion 614, so it is desirable that the through-hole(s)616 have an aperture width of less than or equal to about 2.0 μm inaverage of the positive electrode active material layer 223A. Also, itis desirable that the through-hole(s) 616 have an aperture width ofequal to or greater than 0.01 μm in average of the positive electrodeactive material layer 223A.

In this lithium-ion secondary battery 100A, the positive electrodeactive material particles 610 each have the shell portion 612, thehollow portion 614, and the through-hole(s) 616, as described above.Furthermore, the present inventor proposes that the major axis length L1of the primary particles 800 of the positive electrode active materialparticles 610A should be less than or equal to 0.8 μm. The use of suchpositive electrode active material particles 610A can improve the outputpower of the lithium-ion secondary battery 100A especially in a low SOCregion.

<<Control of Major Axis Length L1 of Primary Particles 800>>

As a result of assiduous studies, the present inventor discovered thatthe major axis length L1 of the primary particles 800 of the positiveelectrode active material particles 610A can be controlled by thesintering temperature and the sintering time, for example, in theabove-described manufacturing method. The present inventor believes thatit is appropriate to carry out the sintering at a sintering temperatureof from about 750° C. to 950° C. for a sintering time of from 5 hours to15 hours. Moreover, the major axis length L1 of the primary particles800 of the positive electrode active material particles 610A can also bevaried by the amount of Li relative to the transition metal (Me)contained in the positive electrode active material particles 610A. Thepresent inventor suggests that the amount of Li relative to that of thetransition metal (Me) be adjusted to be a mole ratio (Li/Me)=1.03 to1.22. In this case, the mole ratio should preferably be set to equal toor greater than 1.05, for example, equal to or greater than 1.07. On theother hand, the mole ratio should preferably be set to less than orequal to 1.20, for example, less than or equal to 1.18.

<<Addition of Tungsten>>

According to the knowledge of the present inventor, it is desirable thattungsten be added to the positive electrode active material particles610A, in order to make the major axis length L1 of the primary particles800 of the positive electrode active material particles 610A less thanor equal to about 0.8 μm.

The present inventor also discovered that, in the case of addingtungsten to the positive electrode active material particles 610A, themajor axis length L1 of the primary particles 800 of the positiveelectrode active material particles 610A can be adjusted by the amountof tungsten added. Specifically, in order to obtain the positiveelectrode active material particles 610A in which the major axis lengthL1 of the primary particles 800 is less than or equal to 0.8 μm with thelayered lithium-transition metal oxide containing nickel, cobalt, andmanganese as its constituent elements as described above, it isdesirable to add 0.05 mol % to 2.0 mol % of tungsten relative to thetransition metals. It is desirable that the amount of tungsten addedshould be, for example, equal to or greater than 0.1 mol %, morepreferably equal to or greater than 0.2 mol % relative to the amount ofthe transition metals. On the other hand, the amount of tungsten addedmay be, for example, less than or equal to 1.5 mol %, or less than orequal to 1.0 mol %. This makes it easy to control the major axis lengthL1 of the primary particles 800 of the positive electrode activematerial particles 610A.

<<Evaluation of Positive Electrode Active Material Particles 610>>

In the following, the present inventor prepared a plurality of types ofpositive electrode active material particles that have different majoraxis lengths L1 of primary particles 800. Test batteries were preparedfor the respective particles to compare their battery performance. Inassociation with their major axis lengths L1 of the primary particles800, the plurality of types of positive electrode active materialparticles prepared here are different in the thickness of the shellportion 612, the particle void fraction, the presence or absence of thethrough-hole(s) 616, the amount of Li relative to that of the transitionmetal, the amount of tungsten added, and so forth.

<<Test Battery>>

Hereinbelow, the structure of the test batteries is described. The testbattery is a flat prismatic battery as illustrated in FIG. 1, and thebasic structure thereof is substantially the same as that of theabove-described lithium-ion secondary battery 100A. For this reason,reference is made to the drawings for the lithium-ion secondary battery100A as appropriate to describe the test battery. For the test battery,the portions and components that exhibit the same functions as those inthe lithium-ion secondary battery 100A are denoted by the same referencesigns.

<<Negative Electrode of the Test Battery>>

As illustrated in FIGS. 1 and 5, the negative electrode of the testbattery has a negative electrode current collector 241 and a negativeelectrode active material layer 243 retained on the negative electrodecurrent collector 241. The negative electrode active material layer 243has negative electrode active material particles 710 and a binder 730.

This test battery uses a copper foil having a thickness of about 10 μmas the negative electrode current collector 241. The negative electrodecurrent collector 241 is a strip-shaped sheet material having a width ofabout 120 mm and a length of about 3200 mm, in which an uncoated portion242 being devoid of the negative electrode active material layer 243 isprovided in one lateral-side edge thereof along the longitudinaldirection. The negative electrode active material layer 243 is retainedon both faces of the negative electrode current collector 241 in theregion except for the uncoated portion 242 (in the region having a widthof about 105 mm).

<<Negative Electrode Active Material Particles of the Test Battery>>

The negative electrode active material particles 710 (see FIG. 5)contained in the negative electrode active material layer 243 aregraphite particles that are prepared by mixing and impregnating 4 mass %of pitch into 96 mass % of natural graphite powder, and sintering theresultant material under an inert atmosphere at 1000° C. to 1300° C. for10 hours. By sieving the thus-obtained graphite particles, the negativeelectrode active material particles 710 used here are adjusted so thatthe average particle size (median diameter D50) thereof falls within therange of from about 8 μm to about 11 μm, and the specific surface areathereof falls within the range of from about 3.5 m²/g to about 5.5 m²/g.

<<Preparation of Negative Electrode Active Material Layer 243>>

The negative electrode active material layer 243 further contains athickening agent. The thickening agent is a material for adjusting theviscosity of the mixture prepared in forming the negative electrodeactive material layer 243. Here, carboxymethylcellulose (CMC) is used assuch a thickening agent. Styrene-butadiene rubber (SBR) is used as thebinder 730 (binder agent).

Here, the negative electrode active material particles 710, thethickening agent, and the binder 730 are kneaded with water in a weightratio of about 98.6:0.7:0.7, to prepare a negative electrode mixture ina paste form (i.e., the negative electrode paste). Then, the negativeelectrode mixture is coated on both faces of the negative electrodecurrent collector 241 except for the uncoated portion 242, and thecoated current collector is dried, to form the negative electrode activematerial layer 243. The resultant article is further pressure-rolled bya roll press machine so that the density of the just-mentioned negativeelectrode active material layer 243 becomes from about 1.0 g/cm³ toabout 1.2 g/cm³. Thus, a negative electrode sheet 240 (see FIG. 2) canbe obtained. Herein, the weight per unit area of the negative electrodeactive material layer 243 after dried was adjusted to about 7.5 mg/cm²in total of both layers on the negative electrode current collector 241.In addition, both layers on the negative electrode current collector 241were adjusted to have substantially the same weight per unit area.

<<Positive Electrode of the Test Battery>>

As illustrated in FIGS. 1 and 6, the positive electrode of the testbattery comprises the positive electrode current collector 221 and thepositive electrode active material layer 223 retained on the positiveelectrode current collector 221. The positive electrode active materiallayer 223 has the positive electrode active material particles 610A, theconductive agent 620A, and the binder 630A (see FIG. 11).

In this test battery, an aluminum foil having a thickness of about 15 μmis used as the positive electrode current collector 221. The positiveelectrode current collector 221 is a strip-shaped sheet material havinga width of about 115 mm and a length of about 3000 mm, in which anuncoated portion 222 being devoid of the positive electrode activematerial layer 223 is provided in one lateral-side edge thereof alongthe longitudinal direction. The positive electrode active material layer223 is retained on both faces of the positive electrode currentcollector 221 in the region except for the uncoated portion 222 (in theregion having a width of about 95 mm).

<<Positive Electrode Active Material Particles of the Test Battery>>

The positive electrode active material particles 610A (see FIG. 11)contained in the positive electrode active material layer 223 areobtained as follows. A mixed solution of nickel sulfate (NiSO₄), cobaltsulfate (CoSO₄), and manganese sulfate (MnSO₄) is neutralized by sodiumhydroxide (NaOH). Then, a transition metal hydroxide that serves as theprecursor is obtained in the step (source hydroxide producing step) ofsupplying ammonium ions (NH⁴⁺) to such an aqueous solution of atransition metal compound to precipitate particles of a transition metalhydroxide from the aqueous solution. This means that Ni, Co, and Mn arecontained approximately in a predetermined ratio in the transition metalhydroxide, which serves as the precursor, in this test battery. Notethat, in the case of obtaining the positive electrode active materialparticles 610A containing tungsten, an aqueous solution a transitionmetal compound containing tungsten was produced in the source hydroxideproducing step in this evaluation test. Then, in the nucleation stage,the transition metal hydroxide containing tungsten was obtained as thetransition metal hydroxide that serves as the precursor.

For this the test battery, lithium carbonate (Li₂Co₃) is mixed with thetransition metal hydroxide, which serves as the precursor, in thepreviously-described mixing step. Then, in the sintering step, theresultant mixture is sintered at about 800° C. to about 950° C. for 5hours to 15 hours. Thus, the positive electrode active materialparticles 610A substantially havingLi_(1.14)Ni_(0.33)Co_(0.33)Mn_(0.33)O₂ as its basic composition werefabricated. As illustrated in FIG. 13, such positive electrode activematerial particles 610A each comprise the shell portion 612, the hollowportion 614, and the through-holes 616. Furthermore, the thus-obtainedpositive electrode active material particles 610A are sieved andadjusted so that the average particle size (median diameter D50) canfall within the range of from about 3 μm to about 8 μm, and the specificsurface area thereof can fall within the range of from about 0.5 m²/g toabout 1.9 m²/g.

Here, according to the above-described manufacturing method, samples ofthe positive electrode active material particles 610A that are differentin the major axis length L1 of the primary particles 800, the thicknessof the shell portion 612, the particle void fraction, and the presenceor absence of the through-hole(s) 616 were prepared by varying theamount of Li relative to the transition metal (mole ratio (Li/Me)), theamount of tungsten added (W amount added), and the sinteringtemperature. In order to compare and analyze to what degree theperformance of the test battery can change, the composition of thepositive electrode active material particles 610A was made generally thesame in the samples except for the amount of Li relative to thetransition metal (mole ratio (Li/Me)) and the amount of tungsten added(W amount added).

<<Preparation of Positive Electrode Active Material Layer 223A>>

The positive electrode active material layer 223A contains the positiveelectrode active material particles 610A, the conductive agent 620A, andthe binder 630A. In this test battery, acetylene black is used as theconductive agent 620A for the positive electrode active material layer223A, and polyvinylidene fluoride (PVDF) is used as the binder 630. Thepositive electrode active material particles 610A, the conductive agent620A, and the binder 630A are mixed in a weight ratio of 90:8:2 andkneaded with N-methyl-2-pyrrolidone (NMP) to prepare a positiveelectrode mixture in a paste form (i.e., positive electrode paste).

Then, the positive electrode mixture is coated on both faces of thepositive electrode current collector 221A except for the uncoatedportion 222A, and the coated current collector is dried, to form thepositive electrode active material layer 223A. The resultant article isfurther pressure-rolled by a roll press machine so that the density ofthe just-mentioned positive electrode active material layer 223A becomesabout 2.1 g/cm³. Thus, a positive electrode sheet 220 (see FIG. 2) canbe obtained. Herein, the weight per unit area of the positive electrodeactive material layer 223A after dried was adjusted to about 11.8 mg/cm²in total of both layers on the positive electrode current collector221A. In addition, both layers on the positive electrode currentcollector 221A were adjusted to have substantially the same weight perunit area.

<<Electrolyte Solution of the Test Battery>>

Next, the electrolyte solution of the test battery will be describedbelow. For this test battery, the electrolyte solution was prepared bymixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), anddimethyl carbonate (DMC) at a ratio (mole ratio) of 3:3:4, anddissolving 1.1 mol/L of LiPF₆ therein. It is also possible to use, asappropriate, an electrolyte solution in which difluorophosphate(LiPO₂F₂) and lithium bis(oxalato)borate (LiBOB), either alone or incombination, are dissolved at a concentration of about 0.01 mol/L to0.03 mol/L in the solvent.

<<Preparation of Test Battery>>

Next, the test battery is fabricated as follows. Generally asillustrated in FIGS. 1 and 2, the positive electrode sheet 220 and thenegative electrode sheet 240 prepared in the foregoing manner arestacked with the separators 262 and 264 interposed therebetween, andwound together. Then, the resultant wound component is pressed in onedirection orthogonal to the winding axis WL (see FIG. 2) and deformedinto a flat shape, to prepare the wound electrode assembly 200. In thewound electrode assembly 200, the uncoated portion 222 of the positiveelectrode sheet 220 and the uncoated portion 242 of the negativeelectrode sheet 240 are exposed at the opposite sides of the separators262 and 264.

In this test battery, the positive/negative capacity ratio calculatedfrom the charge capacity of the positive electrode and the chargecapacity of the negative electrode is adjusted to 1.5 to 1.9.

The battery case 300 of the test battery is generally what is called aprismatic battery case, as illustrated in FIG. 1, and it includes thecase main body 320 and the lid 340. The battery case 300 has aclosed-bottom quadrangular prismatic tubular case main body 320, whichhas a flat rectangular internal space as the space for accommodating thewound electrode assembly 200, and a lid 340 for closing the opening ofthe case main body 320. To the lid 340 of the battery case 300,electrode terminals 420 and 440 are attached. The uncoated portion 222of the positive electrode sheet 220 of the wound electrode assembly 200is connected to the electrode terminal 420. The uncoated portion 242 ofthe negative electrode sheet 240 of the wound electrode assembly 200 isconnected to the electrode terminal 440.

In this test battery, the wound electrode assembly 200 attached to theelectrode terminals 420 and 440 of the lid 340 in this way isaccommodated in the case main body 320. Thereafter, the joint part 322between the case main body 300 and the lid body 340 of the battery case300 is welded by laser welding, the electrolyte solution is filled intothe battery case 300 through a filling port 350 provided in the lid 340,and the filling port 350 is closed.

<<Conditioning>>

Next, the conditioning process, the measurement of the rated capacity,and the SOC adjustment for the test battery constructed in theabove-described manner will be described in that order.

The conditioning process is carried out according to the followingprocedures 1 and 2.

Procedure 1: The test battery is charged with a constant current at 1 Cto 4.1 V and thereafter rested for 5 minutes.Procedure 2: After procedure 1, the test battery is charged with aconstant voltage for 1.5 hours and then rested for 5 minutes.

<<Measurement of Rated Capacity>>

Next, after the above-described conditioning process, the rated capacityis measured for each of the evaluation test batteries at a temperatureof 25° C. and in a voltage range of from 3.0 V to 4.1 V, through thefollowing procedures 1 through 3.

Procedure 1: The test battery is discharged with a constant current at 1C to 3.0 V, then discharged with a constant voltage for 2 hours, andthereafter rested for 10 seconds.Procedure 2: The test battery is charged with a constant current at 1 Cto 4.1 V, then charged with a constant voltage for 2.5 hours, andthereafter rested for 10 seconds.Procedure 3: The test battery is discharged with a constant current at0.5 C to 3.0 V, then discharged with a constant voltage for 2 hours, andthereafter rested for 10 seconds.Rated capacity: The discharge capacity (CCCV discharge capacity)obtained in the discharge process starting from the constant-currentdischarging and finishing with the constant-voltage discharging inProcedure 3 is defined as the rated capacity. In this test battery, therated capacity is about 4 Ah.

<<SOC Adjustment>>

The SOC adjustment is carried out according to the following procedures1 and 2. It is desirable to perform the SOC adjustment after theabove-described conditioning process and the measurement of ratedcapacity. Herein, in order to make the influence of temperature uniform,the SOC adjustment is carried out in a temperature environment at atemperature of 25° C.

Procedure 1: The test battery is charged from 3 V with a constantcurrent at 1 C so as to be in a state of charge of about 60% of therated capacity (60% SOC). Here, the term “SOC” means the state ofcharge.Procedure 2: After procedure 1, the test battery is charged with aconstant voltage for 2.5 hours.This enables the test battery to be adjusted to a predetermined state ofcharge.

Herein, a plurality of samples of the test battery in whichsubstantially only the positive electrode active material particles 610Aare different were prepared to compare and analyze the performance ofthe test battery. In addition, in order to evaluate the output powercharacteristics at low temperature and in a low charge level, the“output power characteristics in a charged state at −30° C. and at a SOCof 27%” were evaluated as the performance of the test battery.

<<Output Power Characteristics in a Charged State at −30° C. and a SOCof 27%>>

The output power characteristics in a charged state at −30° C. and at aSOC of 27% (hereinafter simply referred to as “output powercharacteristics 1”) can be evaluated by the following procedures.

Procedure 1 [SOC adjustment]: As a SOC adjustment, the test battery isadjusted to a SOC of 27% (a battery voltage value of 3.553 V herein) by1 C constant current charge in a temperature environment at roomtemperature (25° C. herein). Next, the test battery is charged with aconstant voltage for 1 hour.Procedure 2 [setting aside for 6 hours at −30° C.]: After procedure 1,the test battery adjusted to a SOC of 27% is set aside for 6 hours in athermostatic chamber at −30° C.Procedure 3 [constant wattage discharge]: After procedure 2, the testbattery is discharged at a constant wattage (W) in a temperatureenvironment of −30° C. At this time, the number of seconds from thestart of the discharge until the battery voltage reaches 2.0 V ismeasured.Procedure 4 [repeat]: While changing the constant wattage dischargevoltage in procedure 3 within the range of 80 W to 200 W, the foregoingprocedures 1 through 3 are repeated. Here, the foregoing procedures 1through 3 are repeated while increasing the constant wattage dischargevoltage in procedure 3 by 10 W at every time, e.g., 80 W at the firsttime, 90 W at the second time, 100 W at the third time, . . . and soforth, until the constant wattage discharge voltage reaches 200 W.Herein, the constant wattage discharge voltage in procedure 3 isincreased by 10 W for each time. In addition to this, the constantwattage discharge voltage in procedure 3 may be increased by a certainwattage for each time (for example, by 5 W for each time, or by 15 W foreach time). It is also possible to decrease the constant wattagedischarge voltage by a certain wattage for each time from 500 W (forexample, by 5 W for each time, by 10 W for each time, or by 15 W foreach time).Procedure 5 [calculation of output power characteristics 1]: Forexample, as illustrated in FIG. 19, the number of seconds it takes toreach 2.0 V, which is measured under the constant wattage condition inProcedure 4, is plotted along the horizontal axis, and the wattage atthat time is plotted along the vertical axis. Then, from theapproximated curve of the plots, the wattage at 2 seconds is calculatedas the output power characteristics 1.

Such output power characteristics 1 indicate the output power that thetest battery can provide in the case where the battery is set aside at alow charged state of about 27% SOC and in an extremely low temperatureenvironment of −30° C. Accordingly, the output power characteristics 1demonstrate that the higher the wattage is, the higher output power thetest battery can produce. Moreover, the output power characteristics 1also demonstrate that the higher the wattage is, the more stably thetest battery can obtain output power even at a low charged stage ofabout 27% SOC.

Table 1 shows the major axis length L1 of the primary particles 800 ofthe positive electrode active material particles 610A, the thickness ofthe shell portion 612, the particle void fraction, the presence orabsence of through-holes, the amount of Li (mole ratio (Li/Me)) relativeto the transition metal (Me), the amount of tungsten added (W amountadded), the sintering temperature, and the just-described output powercharacteristics 1 of the test battery, for a plurality of samples of thetest battery in which substantially only the positive electrode activematerial particles 610A are different.

TABLE 1 Major axis length of Thickness of shell Particle void Through-Amount of Sintering Output power at primary particle portion fractionhole Li/Me W added temperature −30° C. and 27% SoC Sample μm μm % — —mol % ° C. W 1 0.65 2.19 23.7 Yes 1.14 0.5 930 118 2 0.66 1.07 36.4 Yes1.14 0.9 930 129 3 0.66 0.68 45.9 Yes 1.07 0.8 930 135 4 0.38 0.48 68.2Yes 1.14 0.5 800 151 5 0.47 0.51 53.4 Yes 1.14 0.9 850 145 6 0.51 0.4954.5 Yes 1.07 0.8 860 140 7 0.55 0.50 52.1 Yes 1.15 0.7 880 136 8 0.590.51 57.8 Yes 1.15 0.5 880 132 9 0.65 0.52 51.2 Yes 1.09 0.5 900 131 100.68 0.51 48.6 Yes 1.18 0.5 930 128 11 0.75 0.53 50.9 Yes 1.14 0.5 930121 12 0.79 0.54 50.1 Yes 1.07 0.1 950 118 13 0.84 0.55 58.3 Yes 1.120.1 980 115 14 0.85 0.55 54.4 Yes 1.14 0.1 980 112 15 0.88 0.54 57.6 Yes1.05 0.1 970 108 16 0.91 0.57 47.2 Yes 1.14 0 960 101 17 0.95 0.58 46.1Yes 1.04 0 960 90 18 1.04 0.61 43.8 Yes 1.14 0 1000 72 19 0.68 1.69 19.3No 1.14 0.5 930 86 20 0.65 2.81 3.1 No 1.14 0.5 930 68 21 0.66 3.03 3.9No 1.14 0.5 930 81 22 0.61 2.65 2.6 No 1.14 0.5 930 71

As seen from Table 1, when comparing the case where the positiveelectrode active material particles 610A have the through-holes 616 andthe case where they do not, the output power characteristics 1 tend toshow clearly higher values when the through-holes 616 are present.Herein, the presence or absence of the through-holes 616 was confirmedfrom the cross-sectional SEM image of the positive electrode activematerial particles 610A or the cross-sectional SEM image of the positiveelectrode active material layer 223.

When the through-holes are present in a similar fashion, the outputpower characteristics 1 generally tend to show higher values when theparticle void fraction of the positive electrode active materialparticles 610A is higher. Moreover, the output power characteristics 1generally tend to show higher values when the shell portion 612 of thepositive electrode active material particle 610A is thinner. However,there are cases in which differences in the output power characteristics1 are observed even when the samples have approximately the sameparticle void fraction of the positive electrode active materialparticles 610A and approximately the same thickness of the shell portion612 of the positive electrode active material particle 610A.

Concerning this point, when attention is focused on the major axislength L1 of the primary particles 800 of the positive electrode activematerial particles 610A, the smaller the major axis length L1 is, thehigher value the output power characteristics 1 tend to show generally,even when the particle void fraction of the positive electrode activematerial particles 610A and the thickness of the shell portion 612 ofthe positive electrode active material particle 610A are approximatelythe same.

As described above, the lithium-ion secondary battery 100A according toone embodiment of the present invention comprises the positive electrodecurrent collector 221A and the porous positive electrode active materiallayer 223A retained on the positive electrode current collector 221A, asillustrated in, for example, FIGS. 9 to 11. Here, as illustrated in FIG.11, the positive electrode active material layer 223A contains thepositive electrode active material particles 610A, the conductive agent620A, and the binder 630A.

In such a lithium-ion secondary battery 100A, as illustrated in FIG. 13,for example, the positive electrode active material particles 610A eachcomprise the shell portion 612 comprising a lithium-transition metaloxide, the hollow portion 614 formed inside the shell portion 612, andthe through-hole(s) 616 penetrating through the shell portion 612. Inaddition, the primary particles 800 of the lithium-transition metaloxide have a major axis length of less than or equal to 0.8 μm inaverage of the positive electrode active material layer 223A.

The lithium-ion secondary battery 100A can reduce the diffusionresistance of lithium ions especially in the positive electrode, and canimprove the output power particularly at a low charge level (forexample, at a SOC of about 27%) during charge. Furthermore, thelithium-ion secondary battery 100A can keep high output powercharacteristics even at low temperatures (for example, at −30° C.).

In this case, it is desirable that the major axis length of the primaryparticles 800 of the lithium-transition metal oxide be equal to orgreater than 0.2 μm (for example, equal to or greater than 0.4 μm). Thisenables the positive electrode active material particles 610A to haverequired strength.

As one preferable embodiment of this lithium-ion secondary battery 100A,the proportion of the hollow portion 614 is equal to or greater than 23%of the apparent cross-sectional area of the positive electrode activematerial particle 610A, in average of the positive electrode activematerial layer 223A. Furthermore, it is desirable that the thickness ofthe shell portion 612 be less than or equal to 2.2 μm in average of thepositive electrode active material layer 223A.

In this case, with the lithium-ion secondary battery 100A, theproportion of the hollow portions 614 in the apparent cross-sectionalarea of the positive electrode active material particles 610A is equalto or greater than 23% in average of the positive electrode activematerial layer 223; the positive electrode active material particles610A have the through-holes 616 penetrating through the shell portion612; and moreover, the thickness T of the shell portion 612 of thepositive electrode active material particles 610A is very thin (lessthan or equal to 2.2 μm herein). As a result, lithium ions diffuse intothe inner portion of the shell portion 612 (i.e., the inside of theactive material) quickly. Therefore, the lithium-ion secondary batterycan produce high power stably even at a low charge level.

Herein, the thickness T(k) of the shell portion 621 at an arbitraryposition k within an inner surface of the shell portion 612 is definedby the minimum distance from the arbitrary position within the innersurface of the shell portion 612 to an outer surface of the shellportion 612, in an arbitrary cross section of the positive electrodeactive material layer 223A. It is desirable that the thickness of theshell portion 612 of the positive electrode active material particle610A in average of the positive electrode active material layer 223A beobtained by, for example, obtaining the thickness of the shell portion612 of the positive electrode active material particle 610A in aplurality of arbitrary cross sections of the positive electrode activematerial layer 223A, and determining the arithmetic mean value of thethickness of the shell portions 612 of the positive electrode activematerial particles 610A.

In this case, the arithmetic mean value converges by increasing thenumber of cross sections of the positive electrode active material layer223 for obtaining the thickness of the shell portion 612 of the positiveelectrode active material particle 610A, or by increasing the number ofarbitrary positions k in the inner surface of the shell portion 612 forobtaining the thickness of the shell portion 612 of the positiveelectrode active material particle 610A. The phrase “the thickness ofthe shell portion 612 is less than or equal to 2.2 μm in average of thepositive electrode active material layer 223A” means that thejust-mentioned arithmetic mean value is less than or equal to 2.2 μm.

This lithium-ion secondary battery 100 has such a tendency that thegreater the particle void fraction of the positive electrode activematerial particles 610A is, the better the output power characteristicsthereof is. Preferably, the particle void fraction of the positiveelectrode active material particles 610A should be equal to or greaterthan 30%, more preferably equal to or greater than 45%, and still morepreferably equal to or greater than 60%. Furthermore, this lithium-ionsecondary battery 100 has such a tendency that the thinner the shellportion 612 of the positive electrode active material particle 610A is,the better the output power characteristics thereof is. Preferably, thethickness of the shell portion 612 of the positive electrode activematerial particle 610A should be less than or equal to 1.5 μm, morepreferably less than or equal to 1.00 μm, still more preferably lessthan or equal to 0.8 μm, and yet more preferably less than or equal to0.4 μm. Additionally, in order to ensure the durability of the positiveelectrode active material particles 610A in use, it is desirable thatthe thickness of the shell portion 612 be, for example, equal to orgreater than 0.05 μm, more preferably equal to or greater than 0.1 μm.

Furthermore, the lithium-transition metal oxide that constitutes theshell portion 612 of the positive electrode active material particles610A should desirably be a layered compound containing nickel as aconstituent element thereof. Such a lithium-transition metal oxide maybe, for example, a layered compound containing nickel, cobalt, andmanganese as its constituent elements. It is also possible that thelithium-transition metal oxide may be a layered compound contained asLi_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂. In the formula, 0≦x≦0.2, and0.1<y<0.9, 0.1<z<0.4, M is an additive, and 0≦γ≦0.01. Moreover, theadditive M may be at least one additive selected from the groupconsisting of Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F.

As already described above, the lithium-ion secondary battery 100A canreduce the diffusion resistance of lithium ions especially in thepositive electrode, and can improve the output power characteristics atlow temperatures more reliably. Moreover, the lithium-ion secondarybattery 100A can obtain more stable output power even at a low chargelevel more reliably. Therefore, the lithium-ion secondary battery 100Ais suitable for, for example, a lithium-ion secondary battery as avehicle drive battery, which has a high capacity, for example, a ratedcapacity of equal to or higher than 3 Ah. Such a lithium-ion secondarybattery 100 can provide stable output power even at a lower chargelevel, so the battery can operate in a wider range of SOC. As a result,a greater amount of electric power can be obtained from the lithium-ionsecondary battery 100. Such a lithium-ion secondary battery 100 canextend the driving range on a single charge when used as a vehicle drivebattery.

More preferably, the positive electrode active material particles 610Ashould desirably be positive electrode active material particles thatare manufactured by, for example, a method comprising: a sourcehydroxide producing step of supplying ammonium ions to a transitionmetal solution (aqueous solution of a transition metal compound) toprecipitate particles of a transition metal solution from the transitionmetal solution; a mixing step of mixing the transition metal hydroxidewith a lithium compound to prepare an unsintered mixture; and asintering step of sintering the mixture to obtain active materialparticles. Here, the aqueous solution desirably contains at least onetransition metal element that constitutes the lithium-transition metaloxide.

It is desirable that the source hydroxide producing step include anucleation stage of precipitating a transition metal hydroxide from thetransition metal solution and a particle growth stage of growing thetransition metal hydroxide in a condition in which the pH of thetransition metal solution is lower than that at the nucleation stage.

It is also desirable that the source hydroxide producing step is suchthat the pH of the transition metal solution is from 12 to 13 at thenucleation stage, and that the pH of the aqueous solution is from 11 toless than 12 at the particle growth stage. Thus, with the transitionmetal hydroxide that serves as the precursor of the positive electrodeactive material particles 610A, it is possible to obtain particles ofthe transition metal hydroxide in which the density thereof is highernear the outer surfaces thereof than in the inner portions thereof, soit becomes possible to more stably obtain the positive electrode activematerial particles 610A each comprise a thin shell portion 612, a widehollow portion 614, and through-holes 616.

At this time, it is more desirable that the ammonium ion concentrationin the transition metal solution at the nucleation stage be less than orequal to 20 g/L, and that the ammonium ion concentration in thetransition metal solution at the particle growth stage be less than orequal to 10 g/L. On the other hand, it is desirable that the ammoniumion concentration in the transition metal solution at the nucleationstage and at the particle growth stage be equal to or greater than 3g/L.

In such a lithium-ion secondary battery 100, the positive electrodeactive material particles 610A have a characteristic feature. An activematerial particulate material is used for the positive electrode activematerial particles 610A. Herein, it is desirable that the activematerial particulate material comprise, as illustrated in FIG. 13, theshell portion 612 comprising a lithium-transition metal oxide, thehollow portion 614 surrounded by the shell portion 612, and thethrough-hole(s) 616 penetrating through the shell portion 612. Theactive material particulate material is such that the proportion of thehollow portion 614 is equal to or greater than 23% of the apparentcross-sectional area of the active material particle 610, and thethickness of the shell portion 612 is less than or equal to 2.2 μm, inaverage of the active material particles 610 contained in theparticulate material. Herein, the thickness of the shell portion 621 atan arbitrary position within an inner surface of the shell portion 612is defined by the minimum distance from the arbitrary position withinthe inner surface of the shell portion 612 to an outer surface of theshell portion 612, in an arbitrary cross section of the active materialparticle 610.

In addition, the thickness of the shell portion may be equal to orgreater than 0.05 μm, more preferably equal to or greater than 0.1 μm,in average of the active material particles 610 contained in theparticulate material. This improves the durability of the activematerial particles 610, and therefore serves to stabilize theperformance of the lithium-ion secondary battery 100.

As described previously, it is desirable that the lithium-transitionmetal oxide be a layered compound containing nickel as its constituentelement. Moreover, the lithium-transition metal oxide may be a layeredcompound containing nickel, cobalt, and manganese as its constituentelements. The lithium-transition metal oxide may be a layered compoundcontained as Li_(1+x)Ni_(y)Co_(z)Mn_((1-y-z))M_(γ)O₂. In the formula,0≦x≦0.2, 0.1<y<0.9, 0.1<z<0.4, and M is an additive. Furthermore, M maybe at least one additive selected from the group consisting of Zr, W,Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F.

As for such an additive, it is especially preferable that tungsten beadded to the positive electrode active material particles 610A, in orderto obtain the positive electrode active material particles 610A in whichthe major axis length L1 of the primary particles 800 is less than orequal to about 0.8 μm. For example, with the layered lithium-transitionmetal oxide such as described above containing nickel, cobalt, andmanganese as its constituent elements, it is desirable that tungsten beadded in an amount of 0.05 mol % to 2.0 mol % relative to the transitionmetals. It is desirable that the amount of tungsten added should be, forexample, equal to or greater than 0.1 mol %, more preferably equal to orgreater than 0.2 mol % relative to the amount of the transition metals.On the other hand, the amount of tungsten added may be, for example,less than or equal to 1.5 mol %, or less than or equal to 1.0 mol %.This makes it easy to control the major axis length L1 of the primaryparticles 800 of the positive electrode active material particles 610A.

<<Manufacturing Method of Active Material Particles 610>>

A method of manufacturing active material particles 610 comprises: asource hydroxide producing step of supplying ammonium ions to an aqueoussolution of a transition metal compound to precipitate particles of atransition metal hydroxide from the aqueous solution; a mixing steps ofmixing the transition metal hydroxide with a lithium compound to preparean unsintered mixture; and a step of sintering the mixture to obtainactive material particles 610. Here, the aqueous solution contains atleast one transition metal element that constitutes thelithium-transition metal oxide.

It is desirable that the source hydroxide producing step include anucleation stage of precipitating a transition metal hydroxide from theaqueous solution and a particle growth stage of growing the transitionmetal hydroxide in a condition in which the pH of the aqueous solutionis lower than that at the nucleation stage. This makes it possible toobtain active material particles 610 each having a thin shell portion612, a wide hollow portion 614, and through-holes 616 efficiently andstably.

In this case, in the mixing step, for example, the unsintered mixturemay be allowed to contain tungsten in an amount of from 0.05 mol % to2.0 mol % relative to the amount of other transition metal(s), and inthe step of sintering, the mixture may be sintered at a sinteringtemperature of from 750° C. to 950° C. in an air atmosphere, whereby themajor axis length L1 of primary particles can be adjusted to be L1≦0.8μm. In this case, it is particularly preferable that the lithiumcompound used in the mixing step be lithium carbonate.

<<Another Manufacturing Method of Active Material Particles 610>>

As another method of manufacturing active material particles 610, it ispossible that in the source hydroxide producing step, an aqueoussolution of a transition metal compound containing tungsten may beproduced, and a transition metal hydroxide containing tungsten may beobtained in the form of particles of the transition metal hydroxide. Inthis case, it is also preferable that the transition metal hydroxide beallowed to contain tungsten in an amount of from 0.05 mol % to 2.0 mol %relative to the amount of other transition metal(s). In addition, in thestep of sintering, the mixture can be sintered at a sinteringtemperature of from 750° C. to 950° C. in an air atmosphere, whereby themajor axis length L1 of primary particles can be adjusted to be L1≦0.8μm.

For example, it is possible that, in the source hydroxide producingstep, an aqueous solution Aq_(A) containing at least one element of Ni,Co, and Mn may be prepared, an aqueous solution Aq_(C) containingtungsten be may prepared, and the aqueous solution Aq_(A) and theaqueous solution Aq_(C) that have been prepared here may be mixed underan alkaline condition, to produce the aqueous solution of a transitionmetal compound containing tungsten. Thus, with such a layeredlithium-transition metal oxide containing nickel, cobalt, and manganeseas its constituent elements, tungsten can be added stably, and the majoraxis length L1 of primary particles can be easily adjusted to be L1≦0.8μm.

It is sufficient here that the positive electrode active materialparticles such as follows can be obtained in a stable manner thepositive electrode active material particles each comprise a shellportion comprising primary particles of a layered lithium-transitionmetal oxide, a hollow portion formed inside the shell portion, and athrough-hole or through-holes penetrating through the shell portion, andthe primary particles of the lithium-transition metal oxide have a majoraxis length of less than or equal to 0.8 μm. For this reason, the methodof manufacturing method the positive electrode active material particlesis not limited to the above-described method of manufacturing activematerial particles.

Hereinabove, a lithium-ion secondary battery, a particulate material ofactive material particles, and a method of manufacturing active materialparticles according to one preferred embodiment of the invention havebeen described. However, the present invention is not limited to any ofthe embodiments described above.

As described above, the present invention contributes to improvements inthe output power characteristics of lithium-ion secondary batteries.Therefore, the lithium-ion secondary battery according to the presentinvention is suitable for secondary batteries for vehicle-driving powersources that require high capacity and high power, such as batteries fordriving hybrid vehicles, which require high levels of cycle performanceand output power characteristics at high rate, and further, batteriesfor driving plug-in hybrid vehicles and electric vehicles, which requirea particularly high level of capacity. In this case, as illustrated inFIG. 18, for example, the present invention can be suitably utilized inthe form of a battery module, in which a plurality of secondarybatteries are connected and combined, for a vehicle drive battery 1000for driving a motor (electric motor) of a vehicle 1 such as anautomobile. In particular, the lithium-ion secondary battery accordingto the present invention can produce high power stably even at a lowcharge level, so it can withstand the use at a lower charge level.Therefore, the battery can be used efficiently, and at the same time,even when a high level of capacity is demanded, the number of requiredbatteries can be reduced, resulting in a low cost. Moreover, thelithium-ion secondary battery according to the present invention canproduce high power even in a low-temperature environment. Thus, thelithium-ion secondary battery 100 according to the present invention isespecially suitable as a vehicle drive battery 1000.

REFERENCE SIGNS LIST

-   -   1—Vehicle    -   100, 100A—Lithium-ion secondary battery    -   200, 200A—Wound electrode assembly    -   220, 220A—Positive electrode sheet    -   221, 221A—Positive electrode current collector    -   222, 242A—Uncoated portion    -   223, 223A—Positive electrode active material layer    -   225—Gap (void)    -   240—Negative electrode sheet    -   241—Negative electrode current collector    -   242—Uncoated portion    -   243—Negative electrode active material layer    -   245—Gap (void)    -   262, 264—Separator    -   280—Electrolyte solution    -   290—Charger    -   300—Battery case    -   310, 312—Gap    -   320—Case main body    -   322—Joint portion between lid and case main body    -   340—Lid    -   350—Filling port    -   352—Sealing cap    -   360—Safety vent    -   420, 440—Electrode terminal    -   610, 610A—Positive electrode active material particle (active        material particle)    -   612—Shell portion    -   612 a—Inner surface of shell portion    -   614—Hollow portion    -   616—Through-hole    -   620, 620A—Conductive agent    -   630, 630A—Binder    -   710—Negative electrode active material    -   730—Binder    -   1000—Vehicle drive battery    -   WL—Winding axis

1.-19. (canceled)
 20. A method of manufacturing active material particles, comprising: producing a source hydroxide by supplying ammonium ions to an aqueous solution of a transition metal compound to precipitate particles of a transition metal hydroxide from the aqueous solution, the aqueous solution containing at least one of the transition metal elements constituting a lithium-transition metal oxide; mixing the transition metal hydroxide with a lithium compound to prepare an unsintered mixture; and sintering the mixture to obtain the active material particles, wherein in the step of mixing, the unsintered mixture is allowed to contain tungsten in an amount of from 0.05 mol % to 2.0 mol % relative to the amount of other transition metal(s), and in the step of sintering, the mixture is sintered at a sintering temperature of from 750° C. to 950° C. in an air atmosphere, whereby the major axis length L1 of the primary particles is adjusted to be L1≦0.8 μm.
 21. A method of manufacturing active material particles, comprising: producing a source hydroxide by supplying ammonium ions to an aqueous solution of a transition metal compound to precipitate particles of a transition metal hydroxide from the aqueous solution, the aqueous solution containing at least one of the transition metal elements constituting a lithium-transition metal oxide; mixing a lithium compound and the transition metal hydroxide to prepare an unsintered mixture; and sintering the mixture to obtain the active material particles, wherein: in the step of producing a source hydroxide, an aqueous solution of a transition metal compound containing tungsten is produced, and a transition metal hydroxide containing tungsten is obtained as particles of the transition metal hydroxide; and the transition metal hydroxide is allowed to contain tungsten in an amount of from 0.05 mol % to 2.0 mol % with respect to the amount of other transition metal(s), and in the step of sintering, the mixture is sintered at a sintering temperature of from 750° C. to 950° C. in an air atmosphere, whereby the major axis length L1 of the primary particles is adjusted to be L1≦0.8 μm.
 22. The method of manufacturing active material particles according to claim 21, wherein: the step of producing a source hydroxide further comprises the steps of: preparing an aqueous solution Aq_(A) containing at least one element of Ni, Co, and Mn; preparing an aqueous solution Aq_(C) containing tungsten; and mixing the aqueous solution Aq_(A) and the aqueous solution Aq_(C) under an alkaline condition to produce the aqueous solution of a transition metal compound containing tungsten.
 23. The method of manufacturing active material particles according to claim 20, wherein the lithium compound is lithium carbonate. 